x/tools/gopls: clarify details of executeCommand

[stamblerre]

Two key things to consider:

1. Naming convention for commands and constants for tracking these names across the codebase
2. Making it clear that commands operate only on saved files (if we add commands that handle overlays we can address this, but for now, using snapshots in commands doesn't make a lot of sense)
3. Explore how the run commands without adding them to the VS Code Go extension

[stamblerre]

I believe that all of these questions have been resolved. Naming will be addressed as part of #41187.