Add test controller implementation

[vinistock]

VS Code's offers a super nice test interface that we should include in the Ruby LSP.

We already know which tests exist in a file by executing the code lens request, so we can add a client middleware to capture all code lenses that have type: "test" and use those to populate the test controller.

This PR has an initial exploration of how to implement the test controller Shopify/vscode-ruby-lsp#382.

The test interface should

• Allow users to run test through it
• Be updated when someone runs the test using the code lens instead of the test interface