feat: cache date/time descriptor (#124)

.github/CODEOWNERS

@@ -0,0 +1 @@
+*       @BrightspaceUI/gaudi-dev

lib/common.js

@@ -88,6 +88,7 @@ export function validateFormatValue(value) {
 class DocumentLocaleSettings {
 
 	constructor() {
+		this._cache = new Map();
 		this._htmlElem = window.document.getElementsByTagName('html')[0];
 		this._listeners = [];
 		this._overrides = {};
@@ -101,6 +102,7 @@ class DocumentLocaleSettings {
 	set fallbackLanguage(val) {
 		const normalized = this._normalize(val);
 		if (normalized === this._fallbackLanguage) return;
+		this._cache.clear();
 		this._fallbackLanguage = normalized;
 		this._listeners.forEach((cb) => cb());
 	}
@@ -109,6 +111,7 @@ class DocumentLocaleSettings {
 	set language(val) {
 		const normalized = this._normalize(val);
 		if (normalized === this._language) return;
+		this._cache.clear();
 		this._language = normalized;
 		this._listeners.forEach((cb) => cb());
 	}
@@ -123,6 +126,7 @@ class DocumentLocaleSettings {
 				delete val.date.formats.timeFormats;
 			}
 		}
+		this._cache.clear();
 		this._overrides = val;
 		this._listeners.forEach((cb) => cb());
 	}
@@ -131,13 +135,21 @@ class DocumentLocaleSettings {
 		this._listeners.push(cb);
 	}
 
+	getCacheItem(key, provider) {
+		if (!this._cache.has(key)) {
+			this._cache.set(key, provider());
+		}
+		return this._cache.get(key);
+	}
+
 	removeChangeListener(cb) {
 		const index = this._listeners.indexOf(cb);
 		if (index < 0) return;
 		this._listeners.splice(index, 1);
 	}
 
 	reset() {
+		this._cache.clear();
 		this._language = this._languageInitial;
 		this._fallbackLanguage = null;
 		this.overrides = {};