Formatter: Add SourceType to context to enable special formatting for stub files (#6331)

**Summary** This adds the information whether we're in a .py python
source file or in a .pyi stub file to enable people working on #5822 and
related issues.

I'm not completely happy with `Default` for something that depends on
the input.

**Test Plan** None, this is currently unused, i'm leaving this to first
implementation of stub file specific formatting.

---------

Co-authored-by: Micha Reiser <micha@reiser.io>

crates/ruff_python_ast/src/lib.rs

@@ -1,4 +1,5 @@
 use ruff_text_size::{TextRange, TextSize};
+use std::path::Path;
 
 pub mod all;
 pub mod call_path;
@@ -49,3 +50,36 @@ where
         T::range(self)
     }
 }
+
+#[derive(Clone, Copy, Debug, Default, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+pub enum PySourceType {
+    #[default]
+    Python,
+    Stub,
+    Jupyter,
+}
+
+impl PySourceType {
+    pub const fn is_python(&self) -> bool {
+        matches!(self, PySourceType::Python)
+    }
+
+    pub const fn is_stub(&self) -> bool {
+        matches!(self, PySourceType::Stub)
+    }
+
+    pub const fn is_jupyter(&self) -> bool {
+        matches!(self, PySourceType::Jupyter)
+    }
+}
+
+impl From<&Path> for PySourceType {
+    fn from(path: &Path) -> Self {
+        match path.extension() {
+            Some(ext) if ext == "pyi" => PySourceType::Stub,
+            Some(ext) if ext == "ipynb" => PySourceType::Jupyter,
+            _ => PySourceType::Python,
+        }
+    }
+}