Implement JS-esque StateBase.srcCharCodeAt

[hukkin]

Replaces use of StateBase.srcCharCode with the more JS-esque StateBase.srcCharCodeAt that can return None.

I didn't remove StateBase.srcCharCode for ease of migration because mdit-py-plugins use it. But did deprecate it.

Happy to hear what you think @chrisjsewell . Not sure if this is the solution or migration strategy you want, but it's something 😄

I haven't measured performance yet. We may want to do that if this is something we want to proceed with otherwise

EDIT: This PR is in response to discussion in #186

[codecov]

Codecov Report

❌ Patch coverage is 97.89474% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.11%. Comparing base (d6adf66) to head (6323f8b).
⚠️ Report is 104 commits behind head on master.

Files with missing lines	Patch %	Lines
markdown_it/ruler.py	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #190      +/-   ##
==========================================
- Coverage   96.17%   96.11%   -0.07%     
==========================================
  Files          61       61              
  Lines        3267     3267              
==========================================
- Hits         3142     3140       -2     
- Misses        125      127       +2     

Flag	Coverage Δ
pytests	96.11% <97.89%> (-0.07%)	⬇️

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[hukkin]

Seeing that this PR only removes 4 instances of try: ... except IndexError: handling and that almost always we do an if pos < maximum: check or similar to ensure None is not an option, I actually still think it may be a fine decision to reject this change, and simply be more strict/explicit about out-of-range indices than upstream is. The current state is also better for performance. Arguably we'd be throwing away some work that has gone into making upstream more type safe.

The downsides for rejecting are diverging slightly from upstream logic, and the chance that there's still some place in the code where None handling is required but we don't know.