Make grammar less strict

This PR makes the grammar less strict by using optional arguments where possible and avoiding using specific control sequences in dimension references, etc.

There is a lot of other changes listed below.

[x] Remove TeX box commands. These are really difficult to parse and creating errors in valid LaTeX documents.
[x] Add true verbatim environment support. Resolves #45
[x] Add scope based catcodes. Resolves #30
[x] Remove stored catcode commands since group local scopes solves this problem
[x] Add serialization buffer helpers
[x] Move catcode stuff into its own file
[x] Include single EOL in _space token
[x] Add some overflow protections
[x] Alias verbatim bodies to text
[x] Add true support for alltt versus pretending that is a verbatim environment
[x] Allow exit to be produced at EOF

yitzchak / tree-sitter-latex

Make grammar less strict #46