apply suggested fixes

Usage:
  gopls [flags] fix [fix-flags] <filename>

Example: apply fixes to this file, rewriting it:

	$ gopls fix -a -w internal/cmd/check.go

The -a (-all) flag causes all fixes, not just preferred ones, to be
applied, but since no fixes are currently preferred, this flag is
essentially mandatory.

Arguments after the filename are interpreted as LSP CodeAction kinds
to be applied; the default set is {"quickfix"}, but valid kinds include:

	quickfix
	refactor
	refactor.extract
	refactor.inline
	refactor.rewrite
	source.organizeImports
	source.fixAll

CodeAction kinds are hierarchical, so "refactor" includes
"refactor.inline". There is currently no way to enable or even
enumerate all kinds.

Example: apply any "refactor.rewrite" fixes at the specific byte
offset within this file:

	$ gopls fix -a internal/cmd/check.go:#43 refactor.rewrite

fix-flags:
  -a,-all
    	apply all fixes, not just preferred fixes
  -d,-diff
    	display diffs instead of edited file content
  -l,-list
    	display names of edited files
  -preserve
    	with -write, make copies of original files
  -w,-write
    	write edited content to source files