Cleanup after test explorer is rolled out

[vinistock]

After the new test explorer experience is fully rolled out, we have some clean up to do in our code.

• Clean up old test explorer code. This includes the old handlers and the custom code lens middleware to discover tests
• Clean up the code lens implementation. It should no longer be responsible for discovering tests, but instead read the cached result from the document instance
• After cleaning up code lens, we have decoupled the explorer from it and can make showing the Run, Run in terminal and Debug code lens configurable