Enhanced Natural Language Parser Flexibility

This PR improves the natural language parsing capabilities by adding support for more common variations and abbreviations in date/time expressions.

Changes

• Added support for business/work day variations (e.g., "business days", "work days")
• Enhanced weekday parsing to handle more abbreviations (e.g., "Mon.", "Tue.", "Wed.")
• Added support for month abbreviations with periods (e.g., "Jan.", "Feb.", "Sept.")
• Added support for time unit shortcuts (e.g., "hr", "mins")
• Added support for "@" symbol as an alternative to "at" for time specifications
• Added deduplication for repeated weekdays and months in expressions
• Added support for "&" as a list separator alongside "and" and commas
• Improved error messages by including the unknown symbol in the error text

Examples of New Supported Formats

"Every Mon & Wed" → RRULE:FREQ=WEEKLY;BYDAY=MO,WE
"Every business day" → RRULE:FREQ=WEEKLY;BYDAY=MO,TU,WE,TH,FR
"Every day @ 10" → RRULE:FREQ=DAILY;BYHOUR=10
"Every 2 hrs" → RRULE:INTERVAL=2;FREQ=HOURLY

Testing

• Added comprehensive test cases for all new format variations
• Converted test suite to use it.each for better test organization and failure reporting
• No regressions