display selected identifier's references

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

Example:

	$ # 1-indexed location (:line:column or :#offset) of the target identifier
	$ gopls references helper/helper.go:8:6
	$ gopls references helper/helper.go:#53

references-flags:
  -d,-declaration
    	include the declaration of the specified identifier in the results