feat: Code suggestions in unsaved files

Description

It is a small change but it will allow the Language Client to watch unsaved files and send completion requests to the LS.

Related Issues

gitlab-org/editor-extensions/gitlab-lsp#123 (closed)

How has this been tested?

👀 Watch the Demo

Scenario 1

• Link the LS from this MR gitlab-org/editor-extensions/gitlab-lsp!841 (merged)
• Run the extension from this branch
• Create a new Untitled file
• Make sure to set the language mode to the one supported by Duo. How to do set file language
• Trigger code suggestions

Scenario 2

• Go to the User settings > Workbench > Editor: Language Detection (make sure the checkbox is ✔)
• Create a new Untitled file
• Start typing some code in the supported by Duo language
• At some point, VSCode should detect the programming language and Code Suggestions will start working

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