Merge branch 'pr-155' to add support for unary arithmetic operators
including increment (++) and decrement (--) in both prefix and postfix forms.

Resolved conflicts in:
- parser.rs: Unified UnaryArithmeticExpr structure with Pre/Post variants
- execute.rs: Updated evaluation logic for new operator structure
- types.rs: Added unary_op method for pre/post increment/decrement

The implementation now supports expressions like:
- ++a (pre-increment)
- a++ (post-increment)
- --a (pre-decrement)
- a-- (post-decrement)

Fixed the following issues raised in code review:

1. Added validation to reject increment/decrement on literal numbers
   - Now correctly errors on expressions like `echo $(++2)`
   - Operators can only be applied to variables or parenthesized expressions

2. Replaced .unwrap() calls with proper error handling
   - Fixed unwrap() in unary_pre_arithmetic_expr (line 1738)
   - Fixed unwrap() in unary_post_arithmetic_expr (line 1783)
   - Used ok_or_else() with descriptive error messages

3. Fixed missing parse_variable_expansion function
   - Restored function that was accidentally removed during merge
   - Fixes compilation errors

4. Fixed stray conflict marker in types.rs
   - Removed leftover "<<<<<<< HEAD" marker

All tests pass and manual testing confirms:
- Post-increment (a++) returns original value, updates variable
- Pre-increment (++a) returns new value, updates variable
- Invalid expressions like ++2 correctly produce errors