Test Explorer in VS Code Extension and @Test() Attribute

[sezna]

This PR is for enabling VS Code's Test Explorer functionality in our extension, and it introduces the @Test() attribute on callables. It also switches our libraries over to use @Test() instead of the old Main-based pattern.

Any callable that takes no arguments can be annotated as a @Test(). If it does not crash (via Fact or otherwise), it is considered a pass. If it does crash, then it is considered a failure.

After this PR is in, when you open a .qs file, you'll see this icon:

Clicking this icon will take you to the test explorer:

The test explorer shows all discovered tests in their namespace hierarchy. Clicking the "run" play button on a namespace runs all child tests contained in that namespace. A test turns green if its last run passed, or red if it didn't.

Other features:

"Run Test" button next to the test itself

"Failed test" icon on failed tests

Notes:

1. This PR also fixes a bug where the Q# event handler would print [object Object] as the output of a failed run.
2. This PR adds an event emitter to the vscode extension. This event emitter is used to emit events any time updateDocument is triggered. The test explorer listens for this event to refresh tests for a specific URI. The test discovery code has to scan for tests across an entire package, since tests could be added to a namespace from any file due to explicit namespaces. But only the updates from the new URI are actually populated to the test explorer.

Potential future work:

• Support test cancellation tokens
• Support "continuous mode" where we auto-rerun tests based on watching for file changes
• Test coverage tracking (VS Code supports tracking lines of code coverage through the test explorer)
• Debug/Coverage profile modes (currently, only the "Run" profile is supported)
• A "run test" codelens on callables annotated with @Test() (this was deferred for now as I want that experience to integrate with the Debugger service and that requires a bit more work).