Better folding

[James-Yu]

This aims to resolve some new minor issues when the indentation-based folding was replaced by a handicraft folding logic @neilsustc. The new PR makes use of stacks to store env folding, and a line number list to handle section nestings. The env stack part was contributed by @iamhyc #984.

@neilsustc @Astrantia Please take a look.

[James-Yu]

Quite interestingly, @iamhyc 's implementation is quite javascript-ish, while mine is old-fashioned loop-in-loop-in-loop. Refactoring my part is welcomed for anyones OCD's sake.

[yzhang-gh]

(I will do it if it is still not taken by someone else by this weekend 😂)

[Melvasul]

Hello there(hoping that it's the right place to post because I did not want to open a new issue/feature request)

I saw that this extension lets you collapse many sections of the .tex files such as
\chapter \(sub)(sub)section \(sub)paragraph
as well as \begin{...} and so on

but not the section: \part{}
Is it possible to add it?

Thank in advance, have a nice day!