[rush] Integrate phased builds with VS Code

[dmichon-msft]

Summary

Exploratory hack project to control a phased Rush build from a VS Code extension and present the developer with the current build states of all phases in all requested projects.

Should ideally be combined with #3162, as the objectives are orthogonal

Details

Provides a VSCode panel that allows the developer to select one or more Rush projects as in scope, and trigger a worker thread that will watch for file saves and perform incremental phased rush builds, reporting the status of all operations to the UX.

How it was tested

rush build -T vscode-extension
Invoke npm run watch in apps/vscode-extension
Run the provided VSCode launch configuration option
Set "rush.useWorkspaceRushVersion": false in VSCode settings in the launched extension host window.
Ensure that the workspace being tested in is configured to allow the version of node that VSCode is running in
Reload the window (Ctrl+R)
Open the Rush tab