Add translation key getter

Author: NebelNidas

src/main/java/net/fabricmc/mappingio/i18n/Translatable.java

@@ -31,4 +31,11 @@ static Translatable of(String translationKey) {
 	 * Translates this translatable to the specified locale, with a fallback to en_US.
 	 */
 	String translate(Locale locale);
+
+	/**
+	 * Returns the translation key of this translatable, allowing consumers provide a custom translation.
+	 *
+	 * <p>No guarantees about the stability of this value across major library versions are made.
+	 */
+	String translationKey();
 }

src/main/java/net/fabricmc/mappingio/i18n/TranslatableImpl.java

@@ -28,5 +28,10 @@ public String translate(Locale locale) {
 		return I18n.translate(translationKey, locale);
 	}
 
+	@Override
+	public String translationKey() {
+		return translationKey;
+	}
+
 	private final String translationKey;
 }