New test explorer implementation

[vinistock]

Finalizing this issue should also resolve #2639, #3090, #2167, #1528, #1334 and #1155, if they are not resolved otherwise.

What

Revamp our simplistic implementation of the test explorer to allow full discovery of tests, taking add-on contributions into consideration and allowing for multiple tests to be executed within the same command.

How

The idea is to lean on VS Code's ability to lazily resolve test discovery, so that extremely large projects don't lag and don't overwhelm the user with an insane number of tests.

To discover test directories and the test files defined inside of them, we will use a purely client side implementation using the VS Code APIs. When the user expands a specific test file, that will then trigger a custom request to the server, which will gather all add-on contributions and discover every test group and example inside of that file.

When running a group of tests selected by the user, another custom request will be fired to the server, so that add-ons have the opportunity to compose the command that will be executed to run all tests at once. Regardless of test framework, the add-ons need to have the ability to register an LSP JSON formatter that will return the test results in a standardized way, so that processing results for any test framework in the client side is always the same.

• Start discovering all test files lazily Start discovering all available tests lazily #3120
• Add abstraction for representing test items in the server Add test items collection builder #3169
• Implement custom request to discovery all test groups and examples in a file #3171
• Allow add-ons to enhance the new custom request #3172
• Hook the client side and start discovering tests when a file is expanded #3173
• Add created and deleted file watcher to ensure items are properly updated #3174
• Add custom request that will receive a group of test items and compose the command required to run the tests #3175
• Investigate if we can make a generic LSP test reporter that uses JSON to format the output. If not, then just implement one for Minitest and Test Unit #3176
• Ensure that the extension is getting the command to run from the new custom request and that it is populating the results based on the JSON output #3177
• Add support for the Minitest spec style syntax #3178
• Add support for the new API in the Rails add-on #3179