PolyGerrit: Highlight tex files

Add tex files to gr-syntax-layer's language map. Also update hljs to
d24895f4 to include this fix: [1]

  fix(parser) Make language alias registration case insensitive

[1] highlightjs/highlight.js#3026

Bug: Issue 14091
Change-Id: I41f855703f0a9d70b6aebd2a337f1c4c7bf91904

Author: davido