Fix template parsing for nested parentheses

[ziodave]

Issue:
#217

Summary:
This PR replaces the regex-based $(...) parsing with a balanced‑parentheses parser that preserves nested parentheses (e.g., XPath contains(...)) while still handling quotes and escaped brackets. It also applies
the same logic in the expander’s template parsing for watched properties.

What changed:

• Added a balanced template parser in lib/abstract-generator.js, with quote/escape handling and a fallback path for legacy escaped bracket cases.
• Mirrored parsing logic in lib/expander.js for consistency.
• Added a regression test fixture for XPath contains() in test/xpath-contains-template/ and a corresponding test case in lib/rml-generator.test.js.

Tests:

• npm test

Disclosure:
This PR was written by Codex (AI assistant).