Consider removing `workspaceContains` activation event

**Is your feature request related to a problem? Please describe.**

This extension will eagerly activate if I have any workflow files in my repo... even if I don't intend to do any work on GitHub Actions in my repo.

This is because this extension has the following activation event:
https://github.com/github/vscode-github-actions/blob/main/package.json#L28

**Describe the solution you'd like**

Remove this activation event. Your extension will already activate when one of your views open or when you open a workflow file thanks to your [registering of a language](https://github.com/github/vscode-github-actions/blob/main/package.json#L40-L41). This should be sufficient. If it's not, I'm curious to know why.

**Additional context**

Activating eagerly affects other extensions running in the extension host and the ideal situation is that only extensions that are absolutely needed are activated. This keeps VS Code & the extension host lean and fast.