Add an implicit concatenation flag to string and bytes constants (#6512)

## Summary

Per the discussion in
https://github.com/astral-sh/ruff/discussions/6183, this PR adds an
`implicit_concatenated` flag to the string and bytes constant variants.
It's not actually _used_ anywhere as of this PR, but it is covered by
the tests.

Specifically, we now use a struct for the string and bytes cases, along
with the `Expr::FString` node. That struct holds the value, plus the
flag:

```rust
#[derive(Clone, Debug, PartialEq, is_macro::Is)]
pub enum Constant {
    Str(StringConstant),
    Bytes(BytesConstant),
    ...
}

#[derive(Clone, Debug, PartialEq, Eq)]
pub struct StringConstant {
    /// The string value as resolved by the parser (i.e., without quotes, or escape sequences, or
    /// implicit concatenations).
    pub value: String,
    /// Whether the string contains multiple string tokens that were implicitly concatenated.
    pub implicit_concatenated: bool,
}

impl Deref for StringConstant {
    type Target = str;
    fn deref(&self) -> &Self::Target {
        self.value.as_str()
    }
}

#[derive(Clone, Debug, PartialEq, Eq)]
pub struct BytesConstant {
    /// The bytes value as resolved by the parser (i.e., without quotes, or escape sequences, or
    /// implicit concatenations).
    pub value: Vec<u8>,
    /// Whether the string contains multiple string tokens that were implicitly concatenated.
    pub implicit_concatenated: bool,
}

impl Deref for BytesConstant {
    type Target = [u8];
    fn deref(&self) -> &Self::Target {
        self.value.as_slice()
    }
}
```

## Test Plan

`cargo test`

crates/ruff_python_ast/src/nodes.rs

@@ -881,6 +881,8 @@ pub struct DebugText {
 pub struct ExprFString {
     pub range: TextRange,
     pub values: Vec<Expr>,
+    /// Whether the f-string contains multiple string tokens that were implicitly concatenated.
+    pub implicit_concatenated: bool,
 }
 
 impl From<ExprFString> for Expr {
@@ -2463,8 +2465,8 @@ impl std::cmp::PartialEq<usize> for Int {
 pub enum Constant {
     None,
     Bool(bool),
-    Str(String),
-    Bytes(Vec<u8>),
+    Str(StringConstant),
+    Bytes(BytesConstant),
     Int(BigInt),
     Float(f64),
     Complex { real: f64, imag: f64 },
@@ -2472,38 +2474,68 @@ pub enum Constant {
 }
 
 impl Constant {
-    pub fn is_true(self) -> bool {
-        self.bool().is_some_and(|b| b)
-    }
-    pub fn is_false(self) -> bool {
-        self.bool().is_some_and(|b| !b)
-    }
-    pub fn complex(self) -> Option<(f64, f64)> {
+    /// Returns `true` if the constant is a string or bytes constant that contains multiple,
+    /// implicitly concatenated string tokens.
+    pub fn is_implicit_concatenated(&self) -> bool {
         match self {
-            Constant::Complex { real, imag } => Some((real, imag)),
-            _ => None,
+            Constant::Str(value) => value.implicit_concatenated,
+            Constant::Bytes(value) => value.implicit_concatenated,
+            _ => false,
         }
     }
 }
 
-impl From<String> for Constant {
-    fn from(s: String) -> Constant {
-        Self::Str(s)
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct StringConstant {
+    /// The string value as resolved by the parser (i.e., without quotes, or escape sequences, or
+    /// implicit concatenations).
+    pub value: String,
+    /// Whether the string contains multiple string tokens that were implicitly concatenated.
+    pub implicit_concatenated: bool,
+}
+
+impl Deref for StringConstant {
+    type Target = str;
+    fn deref(&self) -> &Self::Target {
+        self.value.as_str()
     }
 }
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct BytesConstant {
+    /// The bytes value as resolved by the parser (i.e., without quotes, or escape sequences, or
+    /// implicit concatenations).
+    pub value: Vec<u8>,
+    /// Whether the string contains multiple string tokens that were implicitly concatenated.
+    pub implicit_concatenated: bool,
+}
+
+impl Deref for BytesConstant {
+    type Target = [u8];
+    fn deref(&self) -> &Self::Target {
+        self.value.as_slice()
+    }
+}
+
 impl From<Vec<u8>> for Constant {
-    fn from(b: Vec<u8>) -> Constant {
-        Self::Bytes(b)
+    fn from(value: Vec<u8>) -> Constant {
+        Self::Bytes(BytesConstant {
+            value,
+            implicit_concatenated: false,
+        })
+    }
+}
+impl From<String> for Constant {
+    fn from(value: String) -> Constant {
+        Self::Str(StringConstant {
+            value,
+            implicit_concatenated: false,
+        })
     }
 }
 impl From<bool> for Constant {
-    fn from(b: bool) -> Constant {
-        Self::Bool(b)
-    }
-}
-impl From<BigInt> for Constant {
-    fn from(i: BigInt) -> Constant {
-        Self::Int(i)
+    fn from(value: bool) -> Constant {
+        Self::Bool(value)
     }
 }
 
@@ -3056,7 +3088,7 @@ mod size_assertions {
     assert_eq_size!(StmtClassDef, [u8; 104]);
     assert_eq_size!(StmtTry, [u8; 104]);
     assert_eq_size!(Expr, [u8; 80]);
-    assert_eq_size!(Constant, [u8; 32]);
+    assert_eq_size!(Constant, [u8; 40]);
     assert_eq_size!(Pattern, [u8; 96]);
     assert_eq_size!(Mod, [u8; 32]);
 }