[Proof of Concept - RFC] Use of RedHat UI testing framework vscode-extension-tester

[bwieger-atlassian-com]

Motivation
We need a way to confirm that our future changes does not break key customer workflows in the UI.

So, we need to have some sort of UI testing framework.

There are 3 frameworks under consideration:

1. @vscode/test-cli & @vscode/test-electron
2. wdio-vscode-service
3. vscode-extension-tester

This PR explores (3) vscode-extension-tester from Redhat

Changes

1. Dependency for vscode-extension-tester
2. Ignoring of generated files
3. Mocha Config
4. Register of npm run command for test:ui
5. A simple test file

How was this tested

1. Ran "npm run test:ui"
2. [TODO] Packaged a ran extension and walked through manual test plan

Some issues to resolve:

1. TS files still not supported
2. It's not headless
3. npm run test:ui is not part of the CI, yet

[sdzh-atlassian]

Ship it! 🚢🚢🚢 Gotta squash and merge though - otherwise commits are WIP