chore: allow gitlab-web-ide scheme for code suggestions

Description

Adds the gitlab-web-ide file scheme to the code suggestions provider.

This scheme was recently added to files opened by Web IDE.

Related Issues

None

How has this been tested?

1. Check out these changes, run the extension in the VS Code extension host.
2. Switch the language server off
   • "gitlab.featureFlags.languageServer": false in settings.
   • Restart the extensions host (this setting only takes effect on startup)
3. Confirm that code suggestions work.

Web IDE:

1. Pull the changes into the Web IDE, confirm that suggestions are present.
   • cd gitlab-vscode-extension
   • git checkout tristan.read/add-gitlab-web-ide-file-scheme
   • cd ..
   • yarn run start:example
2. Confirm that code suggestions work.

Screenshots (if appropriate)

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes