vscode: Add an optional setting for editor.formatOnSaveMode.

VS Code 1.49 introduced a new setting called editor.formatOnSaveMode that
makes Format On Save only format modified lines rather than the entire file:
https://code.visualstudio.com/updates/v1_49#_only-format-modified-text

Add a commented out optional setting that enables that.

Change-Id: I13f3e0eb345c065737fd4dffc413cdd9df8689e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2404345
Auto-Submit: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Commit-Queue: Chase Phillips <cmp@chromium.org>
Reviewed-by: Chase Phillips <cmp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806406}

tools/vscode/settings.json5

@@ -58,6 +58,8 @@
   "ycmd.global_extra_config": "${workspaceFolder}/tools/vim/chromium.ycm_extra_conf.py",
   "ycmd.confirm_extra_conf": false,
 
+  // Optional: Only format modified lines instead of the whole file.
+  // "editor.formatOnSaveMode": "modifications",
   // Optional: Highlight current line at the left of the editor.
   // "editor.renderLineHighlight": "gutter",
   // Optional: Don't automatically add closing brackets. It gets in the way.