Add initial code lens support

[christopher-b]

This expands on Adam's work in #24, providing out-of-the-box code lens support for:

• Jump to view from controller actions
• Run tests
• Run migrations

We filter out lenses that are not currently supported.

There are a few areas this can be improved:

• Support for user customization of commands
• A nicer terminal launch. It will currently launch a new terminal with each test run. It would be preferable to reuse the same terminal buffer
• Debug support 😬

[adam12]

Looks great! Much thanks :)