Feature request: add an flag for strict matching of generated code

[alcortesm]

What version of GCI are you using?

; gci --version
gci version 0.13.4

Feature Request

Would it be possible to have a flag for a more strict matching for generated files?

For example, something like --skip-generated-strict that only matches files with the regular expression ^// Code generated .* DO NOT EDIT\.$ as the first non-comment, non-blank text in the file, as explained in https://pkg.go.dev/cmd/go#hdr-Generate_Go_files_by_processing_source?

Rationale

Right now, the --skip-generated flag instructs gci to ignore files containing any of these strings:

	const (
		genCodeGenerated = "code generated"
		genDoNotEdit     = "do not edit"
		genAutoFile      = "autogenerated file"      // easyjson
		genAutoGenerated = "automatically generated" // genny
	)

Some of these strings create false positives, often appearing in hand-written files (especially the last one, genAutoGenerated).

[ccoVeille]

It looks like a legitimate requests, maybe skip-generated could becomeva flag parameter that accepts coma separated value of header that are accepted.