Add regex engine and persistent data structures

[takeokunn]

Summary

• add a dependency-free regular expression compiler and backtracking matcher
• add structurally shared persistent vectors and immutable persistent maps
• export the runtime APIs and add focused tests

Supported regex scope

• literals, concatenation, alternation, grouping
• *, +, ? repetition
• ., character classes with ranges and negation
• \\d, \\D, \\w, \\s, escaped literals
• ^ and $ anchors, full-match and first-match search

This intentionally does not implement captures, backreferences, lookaround, or other advanced regex extensions.

Verification

• nix run .#test (1060 passed, 0 failed, 0 errored)
• nix flake check (all checks passed)