x/tools/gopls: add go-to-examples and go-to-tests action

[icholy]

What did you do?

Tried to find examples/tests for the identifier under my cursor.

What did you expect to see?

Some type of GoExamples or GoTests command to navigate to relevant example/tests.

What did you see instead?

For examples, I had to open the http://pkg.go.dev and for tests I need to manually search for them.

[icholy]

As @tv42 pointed out on reddit, the go-to-tests action would be difficult to implement because there is no naming convention currently linking tests to types/methods.