Support the \lbrack and \rbrack square-delimiter aliases

[Happypig375]

Add the LaTeX delimiter aliases \lbrack and \rbrack with the same opening/closing atom classes and glyphs as literal [ and ].

Acceptance criteria:

• \lbrack parses as an opening square delimiter and \rbrack as the matching closing delimiter; neither is treated as an ordinary glyph or parenthesis.
• The aliases work in ordinary formulas, scripts, nested groups, and as delimiters after \left and \right.
• Spacing and scalable delimiter selection match literal [ and ] in every math style.
• Serialization chooses one documented canonical spelling and parse → serialize → parse preserves atom structure.
• Focused parser, atom-class, measurement, and rendering tests cover paired and unpaired uses and malformed \left...\right input.

LaTeX defines \lbrack/\rbrack as the opening/closing square-bracket commands and synonyms for [/]: https://tug.ctan.org/info/latex2e-help-texinfo/latex2e.html

Parent backlog: #81. This was identified from the WpfMath parity tracker #145.