fix: default allowedChars when undefined in slugify

[spokodev]

Problem

slugify() merges options with deepClone({ ...this.options, ...opts }), so an explicitly-passed allowedChars: undefined overrides the default with undefined. That undefined is then interpolated into the character-class template literal as the string "undefined":

slugify('Hello World', { allowedChars: undefined }) // => 'e-d'
slugify('Hello World', { allowedChars: null })      // => 'll-l'

The [^${opt.allowedChars}]+ regex becomes [^undefined]+, so only the letters of the word undefined (u n d e f i) survive — Hello World → e-d.

This is easy to hit in real code, where options are often spread from optional config:

slugify(title, { separator: '_', allowedChars: cfg.allowedChars /* may be undefined */ });

Fix

The adjacent separator and ignore lookups on the same call already guard against undefined (opt.separator ?? '-', opt.ignore ?? []); allowedChars was simply missing the same fallback. Add it:

new RegExp(`[^${opt.allowedChars ?? defaultOptions.allowedChars}]+`, 'g')

?? covers both undefined and null; an explicit allowedChars: '' is left untouched (still means "allow nothing").

Tests

Added a case next to the existing separator: undefined / ignore: undefined coverage, asserting allowedChars undefined/null falls back to the default. It fails before the change (expected 'e-d' to be 'hello-world') and passes after. Full suite green (86/86), biome check clean.