Add "allowBreak" support

[WizardMeow]

Mathjax v4 has added support for line breaks. I make an experimental adaptation for this feature (by adding some allowbreak at appropriate positions).

During the testing, I found that if the addOuterBraces flag is turned on, Mathjax will regard the entire formula content as a single texAtom and refuse to perform line breaks on it. To conduct the test, I turned off this flag, and I'm not sure what will happen if I do so.

Currently, this mr is only for test. Waiting the official release of Mathjax V4.