feat: initial gotestsum support and larger refactoring

[fredrikaverpil]

This is just me playing around with using gotestsum so output is readable when attaching to long-running test (hotkey a in the test summary window), like an entire test suite.

This can allow nice output to be shown (when attached) as tests are running. Under the hood, neotest-golang will still parse the JSON output.

To do:

• Clean up and refactoring, so command arguments can be controlled nicely and intuitively.
• If gotestsum is chosen as "test runner", verify it indeed is on $PATH or throw error (possibly fall back to go test).