emacs: restore tag-changes and query bindings for tag hooks

notmuch-before-tag-hook and notmuch-after-tag-hook are supposed to
have access to two dynamic variables, tag-changes and query, but these
were lost with the switch to lexical binding in fc4cda07 (emacs: use
lexical-bindings in all libraries, 2021-01-13).

Add a variant of Emacs's dlet (not available until Emacs 28) and use
it in notmuch-tag to expose tag-changes and query to the hooks.