Support LaTeX square-bracket delimiter aliases

[Happypig375]

Summary

• add \lbrack and \rbrack as opening and closing square-delimiter aliases
• support the aliases both as ordinary atoms and after \left/\right
• keep literal [ and ] as the canonical serialized spelling
• verify aliases are structurally identical to literal brackets in scripts, groups, paired, nested, and one-sided delimiter forms

Testing

• dotnet run --project CSharpMath.Core.Tests -c Release -- -class CSharpMath.Core.AtomTests.LaTeXParserTest -noColor -noAutoReporters — 297 passed
• dotnet run --project CSharpMath.Core.Tests -c Release -- -noColor -noAutoReporters — 1,525 passed
• git diff --check

Compatibility

No public API changes. Existing serialized formulas continue to use literal square brackets.

Fixes #309