Add 3 more vscode command implementations

[jonah-iden]

What it does

This adds the following 3 api commands from vscode:
vscode.executeFoldingRangeProvider
vscode.executeCodeActionProvider
vscode.executeWorkspaceSymbolProvider

How to test

this extension https://github.com/jonah-iden/theia-notebook-selection-test adds 3 new commands named the same as the one added.
executeFoldingRangeProvider executes the command for the active text document
executeCodeActionProvider executes the command for the active text document at the current selection
executeWorkspaceSymbolProvider executes the command with the query "test"

Follow-ups

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

Looks good to me 👍