Admin Page: add custom numberFormat implementation

This would allow us to get rid of the i18n-calypso package; the only reason we keep this package around today is for its #16397 (comment)

This implementation relies on the browser to do all the work, based on the locale given by the browser or by WordPress.

_inc/client/components/number-format/index.js

@@ -0,0 +1,38 @@
+/**
+ * WordPress dependencies
+ */
+import { __experimentalGetSettings } from '@wordpress/date';
+
+/**
+ * Site locale, or browser locale as fallback.
+ *
+ * @returns {string} Formatted Site locale (e.g. `en-US` or `fr-FR`).
+ */
+export const getLocale = () => {
+	const {
+		l10n: { locale },
+	} = __experimentalGetSettings();
+
+	if ( locale ) {
+		// WP uses underscores, but browsers use hyphens.
+		return locale.replace( '_', '-' );
+	}
+
+	// Fallback to the browser locale if necessary.
+	const language = global?.window?.navigator?.language ?? 'en-US';
+
+	return language;
+};
+
+/**
+ * Format a number using the locale in use on the site.
+ *
+ * @param {number} number - The number to format.
+ *
+ * @returns {string} Formatted number.
+ */
+export const numberFormat = number => {
+	const locale = getLocale();
+
+	return new Intl.NumberFormat( locale ).format( number );
+};