apply suggested fixes

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

Example: apply suggested fixes for this file
	$ gopls fix -w internal/lsp/cmd/check.go

fix-flags:
  -a,-all
    	apply all fixes, not just preferred fixes
  -d,-diff
    	display diffs instead of rewriting files
  -w,-write
    	write result to (source) file instead of stdout
