feat: expression-based filtering and computed columns via ex-mode

[mpryor]

Summary

Extend ex-mode (:) with expression support for complex filters and computed columns, enabling queries like latency > 500 && status != 200.

Problem

Current filtering is regex-based on a single column at a time. There's no way to express compound conditions (e.g. "show rows where latency > 500 AND status is not 200") or create computed columns (e.g. "response_time / 1000").

Features

• Expression-based filtering: :filter latency > 500 && status != 200
• Computed columns: :add-column duration_s = latency / 1000
• Support basic arithmetic, comparison, and logical operators
• String functions (contains, startswith, len, upper/lower)
• Access columns by name in expressions

Technical Considerations

• Keep the expression language simple — this is not SQL or Python, it's a lightweight DSL
• Could use Python's ast.literal_eval or a simple expression parser
• Should integrate with the existing ex-mode command system
• Alternatively, consider a subset of Python expressions (like VisiData) or SQL WHERE clauses (like lnav)

Related

• Inspired by lnav's SQLite queries and VisiData's Python expressions
• Complements existing regex-based filtering (f, F, e, E)