Add TypeScript definitions

Partially copied from wikimedia/banana-i18n#25

package.json

@@ -4,6 +4,7 @@
   "description": "Banana Internationalization library",
   "main": "dist/banana-i18n.js",
   "type": "module",
+  "typings": "types/index.d.ts",
   "scripts": {
     "test": " node --experimental-vm-modules node_modules/.bin/jest test/banana.test.js",
     "build": "webpack --mode=production"

types/index.d.ts

@@ -0,0 +1,57 @@
+export class Banana {
+	constructor(locale: string, options?: BananaOptions);
+	locale: string;
+	parser: BananaParser;
+	messageStore: BananaMessageStore;
+	finalFallback: string;
+
+	load(messsageSource: MessageSource, locale?: string): void;
+	i18n(key: string, ...params: ParameterType[]): string;
+	setLocale(locale: string): void;
+	getFallbackLocales(): string[];
+	getMessage(messageKey: string): string;
+}
+
+export interface BananaOptions {
+	messages?: Messages;
+	finalFallback?: string;
+}
+
+export interface Messages {
+	[messageKey: string]: string;
+}
+
+export interface MessageSource {
+	[localizer: string]: Messages;
+}
+
+export type ParameterType = string | object | number | undefined;
+
+export class BananaParser {
+	constructor (locale: string, options?: { wikilinks: boolean });
+	locale: string;
+	wikilinks: boolean;
+	emitter: BananaEmitter;
+}
+
+export class BananaEmitter {
+	constructor(locale: string);
+	emit(node: any, replacements: any): any;
+	concat(nodes: any): string;
+	replace (nodes: any, replacements: any): string;
+	plural(nodes: any): string;
+	gender(nodes: any): string;
+	grammar(nodes: any): string;
+	wikilink(nodes: any): string;
+	extlink(nodes: any): string;
+	bidi(nodes: any): string;
+	formatnum(nodes: any): string;
+}
+
+export class BananaMessageStore {
+	constructor();
+	load(messageSource: Messages, locale: string): void;
+	getMessage(key: string, locale: string): string;
+	hasLocale(locale: string): boolean;
+}
+