Ignore double backslashes (newlines) in Latex (#784)

[rgemulla]

Fixes the roughest issues of #784; see the discussion there.

[rgemulla]

Here is a summary of what this PR fixes and what's still open. I am not sure where to look for fixing the open issues, but I do think that this PR improves handling LaTeX files already substantially. From my point of view, it can be merged.

Now works correctly

Insertion and pair highlighting

()
\(\)
\\()
\\\(\)
\\\(\\\)

Mismatch highlighting:

(\)
\\(\)

Still does not work

Highlighting only shown when point on opening paren:

(\\)

Backspace deletion removes too much (should only remove "(", but removes "\(")

\\(

Prefixes not detected

\emph{abc}

Tags don't work at all (neither triggers nor highlighting):

\begin{quote}
...
\end{quote}

[rgemulla]

In my point of view, this PR can be merged. There are still some problems open with LaTeX, but the PR nevertheless fixes many of the problems regarding backslashes.

[rgemulla]

Not sure why the checks fail; it does not seem to be related to this PR though.

[Fuco1]

Yea, tests are failing again, I think something in cask/evm broke.

I'll try to run this locally. Some additional tests for the new features would also be nice to add.