feat: make vscode extensions configurable when creating a new workspace

[PierrePaul]

The Issue

Fix #42 A few more suggestions for extensions to add to VSCode

How This PR Solves The Issue

This move the hardcoded enabled vscode extensions into a terraform variable. Users can select which vscode extensions to have enabled by default when creating the workspace.

Manual Testing Instructions

• make build
• make test
• make push-template-drupal-core
• Create a new workspace based on the drupal-core template.
  In the creation screen process, select the Github extension

- When the workspace is ready, open VS Code Web - Make sure Github extension is showing up on the left side

[PierrePaul]

Another neat thing with this, you can even update the parameters after the workspace has been created.