show declaration of selected identifier

Usage:
  gopls [flags] definition [definition-flags] <position>

Example: show the definition of the identifier at syntax at offset 44 in this file (flag.FlagSet):

	$ gopls definition internal/lsp/cmd/definition.go:44:47
	$ gopls definition internal/lsp/cmd/definition.go:#1270

definition-flags:
  -json
    	emit output in JSON format
  -markdown
    	support markdown in responses
