fix: Fix panic in string functions and add CEL string extension support (fixes #85)

This commit fixes the panic that occurred when CEL string extension functions
(size, upperAscii, lowerAscii) were called as methods in comprehensions.

Changes:
- Fix size() handler to properly handle both method calls (target) and function calls (args)
- Implement 10 CEL string extension functions with PostgreSQL SQL conversion
  * lowerAscii() → LOWER()
  * upperAscii() → UPPER()
  * trim() → TRIM()
  * charAt() → SUBSTRING()
  * indexOf() → POSITION()
  * lastIndexOf() → REVERSE() logic
  * substring() → SUBSTRING()
  * replace() → REPLACE()
  * reverse() → REVERSE()
- Add error handlers for unsupported functions (split, join, format, quote)
- Add defensive checks to prevent panics in callCasting, visitCallIndex,
  visitCallMapIndex, visitCallListIndex, visitCallUnary
- Add ErrUnsupportedOperation error constant

Tests:
- Add comprehensive test suite in string_functions_test.go
- All 6 failing test cases from issue #85 now pass
- Tests for all implemented string functions
- Tests for error cases and panic prevention
- All tests pass (make test)
- All lint checks pass (make lint)

Fixes #85

🤖 Generated with Claude Code

Co-Authored-By: Claude <[email protected]>

Author: richardwooding