gopls/internal/golang: implement 'Go to TestXxx' CodeLens and Code Action

[ShoshinNikita]

DO NOT SUBMIT

Add a new code lens that annotates function and method declarations
with their corresponding Test, Example, Benchmark, and Fuzz functions.
The same list of test functions can also be accessed through the Source
Action menu.

The algorithm for matching functions with tests works as follows:

1. Generate potential function names from a test name:
   'TestFoo' -> 'Foo' and 'foo', 'Test_foo' -> 'foo'.
2. Match a test with a function that shares the longest common prefix
   with one of the generated names. This allows us to match 'TestDeletePanics'
   with 'Delete' and 'TestDeleteFunc' with 'DeleteFunc' (but not 'Delete').

Jumping to the test declaration is handled by a new command
'source.go_to_test' that uses 'window/showDocument' under the hood.

For golang/go#75030.

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