Initial release of rxtool: a command-line utility for working with regular expressions.

- Test regex patterns against text, extract matches, replace text, split by pattern, and explain regexes.
- Supports named and numbered group captures.
- Allows use of case-insensitive, multiline, and dotall flags.
- Provides JSON output option for extraction.
- Works with stdin pipes or inline input.
- Zero external dependencies required.