x/tools/gopls: add "Replace field accesses by get/set accessor methods" refactoring

[adonovan]

I'm currently trying to make gopls's parsego.File.Cursor field private and make all uses go through an accessor method, and I realize this is this is the kind of this we should have a tool for.

[gabyhelp]

Related Issues

• x/tools/gopls: code action "Extract interface for type" #65721
• x/tools/gopls: code action to extracting an interface for usage #46665
• x/tools/cmd: add tool to convert between functions and methods #11657

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[findleyr]

FWIW, I also made this transformation while experimenting with #76813, and just asked an agent to "make parsego.File.Cursor lazy", and it did it perfectly. Not sure this type of ad-hoc refactoring requires a specialized tool.