feat: add Regex Tester tool

[sg172003]

Problem Statement

Testing and debugging regular expressions requires switching between the browser console and external tools. A client-side regex tester built into DevUtils would streamline this workflow.

Proposed Solution

Add a Regex Tester page with:

• Regex pattern input with flag support (g, i, m, s)
• Test string input with match highlighting
• Match count, groups, and indices display
• Error state for invalid patterns
• Utility functions in src/utils/regexUtils.ts
• Unit tests in src/tests/regexUtils.test.ts

Alternatives Considered

None — JavaScript's native RegExp handles everything needed client-side.