Execute a gopls custom LSP command

Usage:
  gopls [flags] execute [flags] command argument...

The execute command sends an LSP ExecuteCommand request to gopls,
with a set of optional JSON argument values.
Some commands return a result, also JSON.

Available commands are documented at:

	https://github.com/golang/tools/blob/master/gopls/doc/commands.md

This interface is experimental and commands may change or disappear without notice.

Examples:

	$ gopls execute gopls.add_import '{"ImportPath": "fmt", "URI": "file:///hello.go"}'
	$ gopls execute gopls.run_tests '{"URI": "file:///a_test.go", "Tests": ["Test"]}'
	$ gopls execute gopls.list_known_packages '{"URI": "file:///hello.go"}'

execute-flags:
  -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