Add a few commands for amstex

[tornaria]

• \DeclareMathOperator: since the body is in math mode, do not spell check
• new commands: \theoremstyle, \numberwithin (2 args), \subjclas (1 opt + 1 arg): do not spell-check.

[lervag]

Thanks! You even order the highlight groups consistently! :D

One minor thing, but important IMHO: Can you add some examples of the new commands to the test files, test-amsthm.tex and test-amsmath.tex? You might also want to add tests, but the most important thing is to make it easy to manually inspect with e.g. vim -u test-amsthm.vim.

[tornaria]

I added examples with tests for all 5 commands in the PR and for \begin{proof}.

Rebased to master after the commit implementing #2086, everything seems to be working great together.

Thanks!

[lervag]

Thanks! Great work, much appreciated!