New parser - simplified with a tokenizer pass

[Kolaru]

Fix #100

The new parser is easier to understand and should help with issues that require to modify it.

This is breaking because know spaces are always kept, they are discarded in math mode during layouting.

Also $ .. $ are now parsed as a :inline_math TeXExpr, opening the road for proper line breaks.

[codecov-commenter]

Codecov Report

Attention: 20 lines in your changes are missing coverage. Please review.

Comparison is base (cad0b7f) 81.02% compared to head (06b8643) 79.49%.

Files	Patch %	Lines
src/parser/parser.jl	84.04%	15 Missing ⚠️
src/engine/layout.jl	69.23%	4 Missing ⚠️
src/parser/commands_registration.jl	98.11%	1 Missing ⚠️

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #114      +/-   ##
==========================================
- Coverage   81.02%   79.49%   -1.53%     
==========================================
  Files          10       10              
  Lines         585      556      -29     
==========================================
- Hits          474      442      -32     
- Misses        111      114       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.