fix: preserve a multi-character replacement so slug stays idempotent

[spokodev]

Problem

slug preserves a replacement string that is already present in the input, so that re-slugifying an existing slug is a no-op (idempotent). That preservation only works for a single-character replacement, because it tests char.includes(opts.replacement) against one code unit at a time, which can never match a replacement longer than one character.

As a result a multi-character replacement is treated as disallowed punctuation and stripped:

const slug = require('slug')

slug('foo bar', { replacement: '__' })                              // 'foo__bar'
slug(slug('foo bar', { replacement: '__' }), { replacement: '__' }) // 'foobar'  <- separators lost

A single-character replacement round-trips correctly ('_' → idempotent), so this is an inconsistency rather than intended behavior. The README documents replacement as an arbitrary string with no single-character restriction.

Fix

Detect the replacement with a fixed-length look-ahead and advance past it, mirroring the existing multicharmap handling (i += len - 1). This preserves a replacement of any length while leaving single-character and empty replacements unchanged.

Test

Added an idempotence assertion for a multi-character replacement. It fails before the change and passes after. The full test suite is green and c8 --100 coverage is preserved (100%).