Make JavaScript mode autoclose backticks, remove single quote closing
for Lisp modes, and add explicit triple-quote rule to languages supporting
triple quoting.

Issue #3110

Bumping them for every release just adds too much git
noise and history blowup.

As spotted by the extended linter.

And highlight non-builtin symbols as "variable".

Closes #537

Since v2 is the canonical CodeMirror at this point

So that I can put other doc-related stuff there without polluting the
root dir.

It's tiny anyway, no need to force people to include yet another file.

Courtesy of Zi Han

Courtesy of Cliff Koh