Improve edit prediction example capture (#45536)

This PR improves the `edit prediction: Capture Example` in several ways:
* fixed bugs in how the uncommitted diff was calculated
* added a `edit_predictions.examples_dir` setting that can be set in
order to have the action automatically save examples into the given
folder
* moved the action into the `edit_predictions` crate, in preparation for
collecting this data passively from end users, when they have opted in
to data sharing, similar to what we did for Zeta 1

Release Notes:

- N/A

crates/buffer_diff/src/buffer_diff.rs

@@ -314,6 +314,12 @@ impl BufferDiffSnapshot {
         self.inner.hunks.is_empty()
     }
 
+    pub fn base_text_string(&self) -> Option<String> {
+        self.inner
+            .base_text_exists
+            .then(|| self.inner.base_text.text())
+    }
+
     pub fn secondary_diff(&self) -> Option<&BufferDiffSnapshot> {
         self.secondary_diff.as_deref()
     }