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/all.rs

@@ -24,7 +24,7 @@ where
     fn add_to_names<'a>(elts: &'a [Expr], names: &mut Vec<&'a str>, flags: &mut DunderAllFlags) {
         for elt in elts {
             if let Expr::Constant(ast::ExprConstant {
-                value: Constant::Str(value),
+                value: Constant::Str(ast::StringConstant { value, .. }),
                 ..
             }) = elt
             {