feat: implement brace expansion as strict subset of bash

[wolfv]

This commit adds brace expansion support to the shell, implementing a strict subset of bash's brace expansion functionality.

Supported features:

• Comma-separated lists: {a,b,c} → a b c
• Numeric sequences: {1..10} → 1 2 3 4 5 6 7 8 9 10
• Character sequences: {a..z} → a b c ... z
• Reverse sequences: {10..1} → 10 9 8 7 6 5 4 3 2 1
• Step sequences: {1..10..2} → 1 3 5 7 9
• Reverse step sequences: {10..1..2} → 10 8 6 4 2
• Empty elements: {a,,b} → a b
• Quoted braces don't expand: "{a,b,c}" → {a,b,c}

Implementation details:

• Added BRACE_EXPANSION grammar rule to grammar.pest
• Added BraceExpansion enum to parser.rs with List and Sequence variants
• Added WordPart::BraceExpansion variant
• Implemented expand_braces() and expand_sequence() functions
• Added comprehensive test suite in brace_expansion.sh

The implementation is a strict subset of bash, ensuring compatibility while keeping the implementation simple and maintainable.

Fixes #242