Place DO NOT EDIT comment on top of generated fourslash tests

[jakebailey]

These files are not meant to be edited by hand. Placing this special string at the top tells all Go-knowing tooling that these are generated and should be ignored, hides their diffs in GitHub diffs, tells gopls to issue a warning when you touch these files, and probably helps Copilot know it shouldn't touch these files in general.

[gabritto]

Did the conversion script fail to format?

[jakebailey]

I think it did, because these are generated files and so are not formatted, I guess.

I don't think this is worth the churn. Probably we should just update the copilot instructions.