Allow adding files to .gitignore (#38089)

This feature allows users to add a new, untracked file to `.gitignore`
by using the context menu in the git panel.

<img width="300" alt="Demo screen shot"
src="https://github.com/user-attachments/assets/3f2402fb-9337-42f8-939f-dac12ca09518"
/>

Release Notes:

- Added feature to add a new file to `.gitignore`

crates/git/src/git.rs

@@ -94,6 +94,8 @@ actions!(
         OpenModifiedFiles,
         /// Clones a repository.
         Clone,
+        /// Adds a file to .gitignore.
+        AddToGitignore,
     ]
 );