Support running controversial/experimental rules individually

[maxbrunet]

Certain rules are preferred to be excluded from the global formatting, but it can be interesting to suggest them as individual quick fixes. Providing an API to run them individually would allow the creation of specific code actions in gopls (golang/go#56403)

There are at least 2 potential candidates at the moment:

gofumpt/format/format.go

Lines 645 to 648 in 70d7433

	// Merging adjacent fields (e.g. parameters) is disabled by default.
	if !f.ExtraRules {
	break
	}

gofumpt/format/format.go

Lines 755 to 759 in 70d7433

	if os.Getenv("GOFUMPT_SPLIT_LONG_LINES") != "on" {
	// By default, this feature is turned off.
	// Turn it on by setting GOFUMPT_SPLIT_LONG_LINES=on.
	return
	}

[mvdan]

Worth noting that #78 is related, but here we just need the "extra" rule to be an analysis. The regular formatting could stay the same.