Add mathparpagebreakable as a math environment

[ahfriedman]

Building on #1337, mathpartir also offers a mathparpagebreakable environment in addition to mathpar. As mathpar is already treated as a math environment, for consistency, I think it would make sense to treat the page breakable version as a math environment so that the following is not needed in .vimrcs:

let g:vimtex_syntax_custom_envs = [
      \ {
      \   'name': 'mathparpagebreakable',
      \   'math': v:true,
      \ }
      \]

[lervag]

Agreed.