[Markdown] reduce backtracking possibilities

[keith-hall]

this PR reduces the number of backtracking possibilities for regex engines like Oniguruma that use backtracking, and also explains the regex patterns better.

a simple Markdown snippet with a missing end backtick like

``he`lloworld`

will now fail in 73 steps rather than over 5000!

[FichteFoll]

I find the majority of these comments to be redundant.

I can see myself that [ ]{,3} matches "between 0 to 3 spaces" and do not need a comment to explain that. This is exactly like adding a comment to i += 1 # increase i by 1. Comments should be used to convey information that cannot immediately be seen, such as (?!`) # makes the {3,} possessive (although it appears that this pattern specifically is redundant because the preceding match already ensures that there can't be any following backticks (ref)).

[keith-hall]

I quite like the idea of being able to just read the comments to understand what a regex is doing, without having to look at the pattern itself at all, hence the "redundant" comments :) if you were to become the main maintainer of this package/syntax then I'm sure you could feel free to remove them ;)

I've updated the code fence regex you didn't like though :)

[FichteFoll]

Of course, I was just voicing my opinion and giving an analogy to code.

I just spotted the code fence regex while trying to look for a comment that does not literally describe the regex and something seemed off, so that was pure coincidence. 😆

[wbond]

Everything passes with the latest unreleased version of Sublime Text