toRomaji custom mapping not always respected

[gmarty]

Issue Type

• Bug (https://github.com/WaniKani/WanaKana/blob/master/.github/CONTRIBUTING.md#bug-reports)
• Feature (https://github.com/WaniKani/WanaKana/blob/master/.github/CONTRIBUTING.md#feature-requests)

Description

I an trying to use WanaKana to transliterate hiragana to romaji using the Hepburn romanisation system.
I have configured a custom mapping. There are cases where the custom mapping is not respected.

Steps to reproduce

const customRomajiMapping = {
  うう: 'û',
  おう: 'ô',
  くう: 'kû',
  こう: 'kô',
  すう: 'sû',
  そう: 'sô',
  つう: 'tsû',
  とう: 'tô',
  ぬう: 'nû',
  のう: 'nô',
  ふう: 'fû',
  ほう: 'hô',
  むう: 'mû',
  もう: 'mô',
  ゆう: 'yû',
  よう: 'yô',
  ろう: 'rû',
  ろう: 'rô',
  ぐう: 'gû',
  ごう: 'gô',
  ずう: 'zû',
  ぞう: 'zô',
  づう: 'zû',
  どう: 'dô',
  ぶう: 'bû',
  ぼう: 'bô',
  ぷう: 'pû',
  ぽう: 'pô',
  きゅう: 'kyû',
  きょう: 'kyô',
  しゅう: 'shû',
  しょう: 'shô',
  ちゅう: 'chû',
  ちょう: 'chô',
  にゅう: 'nyû',
  にょう: 'nyô',
  ひゅう: 'hyû',
  ひょう: 'hyô',
  みゅう: 'myû',
  みょう: 'myô',
  りゅう: 'ryû',
  りょう: 'ryô',
  ぎゅう: 'gyû',
  ぎょう: 'gyô',
  じゅう: 'jû',
  じょう: 'jô',
  ぢゅう: 'jû',
  ぢょう: 'jô',
  びゅう: 'byû',
  びょう: 'byô',
  ぴゅう: 'pyû',
  ぴょう: 'pyô',
}
name = toRomaji('こういち', { customRomajiMapping }) // Returns 'Kôichi' (correct)
name = toRomaji('いっこう', { customRomajiMapping }) // Returns 'Ikkou'
name = toRomaji('ゆうじ', { customRomajiMapping }) // Returns 'Yûji' (correct)
name = toRomaji('けんゆう', { customRomajiMapping }) // Returns 'Ken'yuu'

It seems that 'っ' or an apostrophe before the pattern trips it and the transliteration fails.

Versions

• Browser: Node 20
• OS/Device: Mac
• Keyboard/Input Method: UTF-8