Fix duplicate singleton subtag being treated as valid

[youdie006]

Fixes #23.

Problem

parse('en-a-bbb-a-ccc') returns a fully valid schema even though the singleton a appears twice. Per RFC 5646, each singleton subtag MUST appear at most once (other than in a private-use sequence), so this tag is invalid.

Root cause

The extensions loop in lib/parse.js pushes each extension without ever checking whether its one-character singleton was already used, so a tag with the same singleton twice parses as valid -- no warning, and the end-of-input check passes.

Fix

Track the singletons seen while walking the extensions loop; on a repeat, return the existing fail(..., 7, 'Duplicate singleton, singletons must not be repeated'). This reuses the same fail() / warning-code convention the sibling spec violations already use (empty extension, superfluous content), and adds a warning -- matching the steer in #23 that "a warning is probably a good idea at least". Warning code 7 is added to the readme table.

The check is scoped to extension singletons only, so cases that stay valid per RFC are unaffected and covered by regression tests:

• en-a-bbb-x-a-ccc -- the second a is in a private-use sequence (valid)
• en-a-bbb-t-ccc -- two different singletons (valid)
• de-DE-u-co-phonebk -- a normal extension (valid)

Verification

• Red/green: reverting the fix makes the new Duplicate singleton test fail (not ok); with the fix, all tests pass.
• npm run test-api: 10/10 pass.
• Coverage gate c8 --100: 100% across all files.
• tsc build, 100% type-coverage, prettier, and xo all clean.

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This change was written with AI assistance and reviewed by me before submitting.