This feature is loosely based on what 63a29a1
described, but copies [commonmark-hs] more closely (the balanced braces
feature is added).

[commonmark-hs]: https://github.com/nschloe/github-math-bugs

It largely ignores GitHub, because its math parsing [is very buggy].

[is very buggy]: https://github.com/nschloe/github-math-bugs

This approach, based on @ollpu's suggestion, tracks single `$`s
in the inline tree, and merges them later. It avoids having
to merge and unmerge them in some corner cases.

The essential problem is: every time you write `$$x$}`, you get another
entry added to a hash table. Even if it's not [theoretically] *quadratic*,
it's still slow. Hard limiting it to 255 entries makes this not a problem.

Interestingly enough, when I tried to write an analogous torture test
for code spans, I couldn't find a way to do it because code spans are
keyed by their *length* instead of their *position*. In order to get
N entries in the hash table, I basically had to write N `` ` `` in a
row, forcing me to write quadratic amounts of input text.

Comparison:

```
michaelhowell@Michael-Howells-Macbook-Pro pulldown-cmark % python3 -c 'print("$$x$}"*5000)' | time target/release/pulldown-cmark.old -M > /dev/null
target/release/pulldown-cmark.old -M > /dev/null  2.63s user 0.02s system 99% cpu 2.673 total
michaelhowell@Michael-Howells-Macbook-Pro pulldown-cmark % python3 -c 'print("$$x$}"*5000)' | time target/release/pulldown-cmark.new -M > /dev/null
target/release/pulldown-cmark.new -M > /dev/null  0.01s user 0.00s system 6% cpu 0.109 total
```

[theoretically]: http://www.ilikebigbits.com/2014_04_21_myth_of_ram_1.html

Co-authored-by: Linda_pp <rhysd@users.noreply.github.com>

This changes things so that `$$ $ $$` is not parsed as display math.
Doing that doesn't actually make sense, since it's going to make a
parse error at the end anyway.

https://pandoc.org/try/?params=%7B%22text%22%3A%22%24%24+%24+%24%24%22%2C%22to%22%3A%22html5%22%2C%22from%22%3A%22commonmark_x%22%2C%22standalone%22%3Afalse%2C%22embed-resources%22%3Afalse%2C%22table-of-contents%22%3Afalse%2C%22number-sections%22%3Afalse%2C%22citeproc%22%3Afalse%2C%22html-math-method%22%3A%22plain%22%2C%22wrap%22%3A%22auto%22%2C%22highlight-style%22%3Anull%2C%22files%22%3A%7B%7D%2C%22template%22%3Anull%7D

- Disallow $$ matching a closing $ and then marching delimiters in
  `make_math_span`. Instead, retry scanning at the second position.

- Remove the `seen_first` optimization from `MathDelims`. It doesn't
  work with the retry strategy.

Co-authored-by: Michael Howell <michael@notriddle.com>