Change snippet triggers for TeX snippets to be internally consistent

[cxr6548]

I don't like that display math is triggered by 'dm' and inline math is triggered by 'inmath'

Personally I'm more attached to descriptive snippets that are also less likely to have collisions than a two-character trigger but I understand why one would also want something like 'dm' for display math. More important though is having consistency. Either changing 'inmath' to 'im' (seems likely to collide) or maybe both to d$ and in$ would be fine(prefer the latter)

I think line 196 snippet UU "cup" w and line 200 snippet uuu "bigcup" w are a good example as well.

I'd like some input before I open pull requests and step on peoples' toes and more importantly change someone's workflow. Maybe duplicate the existing snippets with new triggers and add a text prompt(auto disappearing) that warns of future deprecation for the old ones?