tildify.el: Optimise environments regexes

tildify.el: Optimise environments regexes

* lisp/textmodes/tildify.el (tildify-ignored-environments-alist):
Each time beginning of an environment to ignore is found,
`tildify-find-env' needs to identify regexp for the ending
of the environment.  This is done by trying all the opening
regexes on matched text in a loop, so to speed that up, this
loop should have fewer things to match, which can be done by
using alternatives in the opening regexes.

Coincidentally, this should make matching of the opening
regexp faster as well thanks to the use of `regexp-opt' and
having common prefix pulled from many regexes.