feat: add open tabs context setting

Description

We're adding an "Open Tabs Context" setting in the VSCode editor that will drive whether the end user sends the context to the LSP. This MR adds the setting.

Related Issues

Related to gitlab-org/editor-extensions/gitlab-lsp#276 (closed)

How has this been tested?

1. Run the GitLab LSP using the branch from this MR that allows the LSP to receive the openTabsContext setting.
2. Run the VSCode extension using the branch from this MR.
3. Validate that the shouldUseAdvancedContext evaluates to true.
4. Validate that the setting appears in VSCode

Screenshots (if appropriate)

Screenshot of the setting as presented in the VSCode Settings UI

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