Add user interface for running unit tests

[rvanasa]

Integrates a Motoko unit test runner directly into the VS Code extension.

This workflow is compatible with testing libraries such as Motoko Matchers and supports running test files via the moc interpreter or as a WASI module.

It's also worth noting that this works completely out of the box using moc.js and Wasmer, both of which are bundled with the extension. As a result, it's no longer necessary to install wasmtime (or even dfx) to locally run the base library unit tests.

Progress:

• [x] Register tests from *.test.mo files in the current workspace
• [x] Show unit tests in VS Code's test explorer UI
• [x] Implement test runner using moc interpreter
• [x] Add support for WASI tests via wasmer
• [x] Configure each test file individually using annotation comments such as // @testmode wasmer

Next steps:

• Distribute as a command-line interface for use outside of VS Code (high priority)
• Recognize multiple independent tests per file when exporting a module { ... } with a function for each test case
• Speed up or cache WASI module compilation
• Modify the moc.js interpreter to distinguish errors from warnings (currently always reports a successful run)
• Identify various stack overflow conditions in the moc.js interpreter and WASI compiler

Feedback is welcome!

[rvanasa]

Corresponding base library PR: https://github.com/dfinity/motoko-base/pull/527