fix: discard slash extglob alternatives

[pupuking723]

Fixes #274.

This updates extglob path-part splitting so slash-containing alternatives are discarded instead of causing the whole extglob pattern to be treated as literal text.

Examples covered:

• @(x|a|a/b)/* keeps the x and a alternatives and drops a/b
• @(a|a/b)/* still matches a/*
• @(a/b)/* remains non-matching instead of broadening the pattern

Verification:

• npx prettier --check src/index.ts test/basic.js
• npx oxlint src test
• npm test