fix: flags without an alias incorrectly reported as unknown in strict mode

[xxiaoxiong]

Summary

Fixes strict mode (unknown callback) incorrectly reporting flags that have no alias as "unknown" when they are listed in string or boolean options.

Changes

• src/index.js: Extended keys array to include opts.boolean and opts.string entries alongside alias keys

Why

In strict mode, keys was built only from opts.alias entries. A flag like --bar listed in string: ['bar'] with no corresponding alias would be rejected as unknown, even though it's a valid recognized flag.

Repro:

mri(['--bar'], {
  string: ['foo', 'bar'],
  alias: { f: 'foo' },
  unknown(flag) { console.error('Unknown:', flag); process.exit(1); }
})
// Before: "Unknown: --bar"
// After: parses correctly -> { _: [], bar: 'true' }

Closes #27