feat: option to strictly follow Go autogenerated file convention

[ldez]

The current autogenerated file detection is very lax and it's expected (#48, #72).

But in some cases, this can be a problem.

Then I added an option to strictly follow Go autogenerated file convention.

To convey to humans and machine tools that code is generated, generated source should have a line that matches the following regular expression (in Go syntax):

^// Code generated .* DO NOT EDIT\.$

This line must appear before the first non-comment, non-blank text in the file.
https://go.dev/s/generatedcode

The strict mode can be extended with simple run.skip-dirs or run.skip-files.

One commit, one change.

Fixes #4506
Fixes #2254

[bombsimon]

LGTM!

Would you like to deprecate the old variant at some point? It feels like supporting anything that is not the Go convention is a bad idea and if you want to do that it would make more sense to use skip-dirs or skip-files but I also see the convenience keeping it and having the strict version as opt-in.

[ldez]

Would you like to deprecate the old variant at some point?

Maybe just change the default (but it's breaking), because I think some people rely on that.

it would make more sense to use skip-dirs or skip-files

I think skip-dirs or skip-files should be deprecated and replaced by something in the issues section with a relevant name.
Because skip-xxx are skipping nothing 😄