Support aliases in glossary directives

[agoose77]

Proposal

Sphinx's glossary directive supports aliases:

.. glossary::

   Term A
   Term B
     This is a definition

It would be nice to support this feature without introducing new esoteric syntax that only works in glossaries.

@rowanc1 and I had a meeting in which the following syntax was proposed:

:::{glossary}
Term A
: This is a definition

Term B
: {term}`Term A`
:::

This uses the {term} role to indicate that Term B is an alias of Term A.