Support Jupytext-style line comments for REPL evaluation ranges (#15073)

This adds support for detecting line comments in the
[Jupytext](https://jupytext.readthedocs.io/) format. When line comments
such as `# %%` is present, invoking `repl: run` will evaluate the code
between these line comments as a unit.

/cc @rgbkrk 

```py
# %%
# This is my first block
print(1)
print(2)

# %%
# This is my second block
print(3)
```

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Thorsten <thorsten@zed.dev>
Co-authored-by: Thorsten Ball <mrnugget@gmail.com>

crates/repl/src/session.rs

@@ -377,7 +377,12 @@ impl Session {
         self.blocks.clear();
     }
 
-    pub fn execute(&mut self, code: &str, anchor_range: Range<Anchor>, cx: &mut ViewContext<Self>) {
+    pub fn execute(
+        &mut self,
+        code: String,
+        anchor_range: Range<Anchor>,
+        cx: &mut ViewContext<Self>,
+    ) {
         let Some(editor) = self.editor.upgrade() else {
             return;
         };
@@ -387,7 +392,7 @@ impl Session {
         }
 
         let execute_request = ExecuteRequest {
-            code: code.to_string(),
+            code,
             ..ExecuteRequest::default()
         };