[docs] Update extension-related instructions for VS Code editor.

This CL mainly includes the following changes:

1. Update the "Useful Extensions" for VS Code. ChromiumIDE-specific ones
   are added, and outdated extensions are removed.
2. Polish the "Setup for Chromium" section. Now it suggests copying all
   VS Code configurations from the Chromium checkout.
3. Add a list of recommended VS Code extensions into
   `tools/vscode/extensions.json`, such that users can just install them
   all from the VS Code UI.
4. Update `tools/vscode/settings.json` to include a recommended C/C++
   configuration from the doc.

Change-Id: Iea24a9203989ecab5248836f72e0cc369fa46c9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5364766


Reviewed-by: Chase Phillips <cmp@chromium.org>
Commit-Queue: Chase Phillips <cmp@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1271713}