Add flexible builtin variables system with $now Date instance

[geclos]

This PR introduces a generic system for builtin variables that resolve at runtime, starting with $now which returns a Date instance.

Changes

• Implemented a builtins object passed through the compilation pipeline
• Modified mustache parsing to always treat expressions uniformly, removing special handling
• Added error checking to prevent assignment to builtin variables
• Updated $now to return a Date instance instead of ISO string, allowing method calls like $now.getTime()
• Added comprehensive tests for builtin usage in expressions and error cases

Features

• Builtin variables can be used anywhere in expressions: {{ time = $now }}, {{ $now.getTime() }}
• Attempting to assign to builtins raises a compile-time error
• System is extensible: add new builtins by updating SPECIAL_IDENTIFIERS and SPECIAL_RESOLVERS in constants.ts

All tests pass with 235/238 (3 todo remaining).