New package, gomodifytags-1.16.0.

This is needed by the new gopls.

gomodifytags is a Go tool to modify/update field tags in structs. It makes it
easy to update, add or delete the tags in a struct field. You can easily add
new tags, update existing tags (such as appending a new key, i.e. db, xml,
etc.) or remove existing tags. It also allows you to add and remove tag
options. It's intended to be used by an editor, but also has modes to run it
from the terminal.

(bsiegert)