syntax: Add REUSE: word to easily USE: but reload.

syntax: Add REUSE: word to easily USE: but reload.

My use case is specifically for load-all with editors. The last
editor on macOS is Xcode and I use vscode, and once you have
``USE: editors.visual-studio-code`` then the next time it's a no-op.

``REUSE: editors.visual-studio-code`` is the same thing but with
two extra characters, as opposed to ``"editors.visual-studio-code" reload``
which requires a lot more typing.

We could have ``EDIT:``, ``EDIT-DOCS:``, ``EDIT-TESTS:`` etc if this REUSE:
word is successful.