[SuperEditor] - Allow spaces in stable tags

[matthew-carroll]

We'd like to support the ability to search for stable tag values across words. For example, we'd like to be able to type "@ma c" to search for "Matt Carroll". The first step to achieving this goal is to allow spaces in stable tags.

Currently stable tags have a concept of prohibited characters, which are used to short circuit the matching process. We should retain this ability for now, but we should also add a way to allow spaces.

The trick to achieve this behavior is to watch for inserted characters instead of re-calculating tag bounds on every reaction.