fix: fix RelativeTimeFormat type definition (#39661)

* fix: fix RelativeTimeFormat type definition
Changes:
1. Change BCP47LanguageTag to UnicodeBCP47LocaleIdentifier: Those mean 2
different things. BCP47LangTag allows _ as separator while UTS35
doesn't. It also allows grandfathered locales and UTS35 doesn't.

2. Combine RelativeTimeFormat interface and const declaration into a
single class. The old way of declaring as `interface` & `const` permits
calling `Intl.RelativeTimeFormat` without `new` which is no longer
possible after `Intl.DateTimeFormat` & `Intl.NumberFormat`. The spec
explicitly forbids it in
http://ecma-international.org/ecma-402/7.0/index.html#relativetimeformat-objects
where:

> If NewTarget is undefined, throw a TypeError exception.

Intl.RelativeTimeFormat is also extensible per spec. This is closer to a
`class` than the current declaration.

* address feedbacks

Author: longlho

src/lib/es2020.intl.d.ts

@@ -1,13 +1,12 @@
 declare namespace Intl {
 
     /**
-     * [BCP 47 language tag](http://tools.ietf.org/html/rfc5646) definition.
+     * [Unicode BCP 47 Locale Identifiers](https://unicode.org/reports/tr35/#Unicode_Language_and_Locale_Identifiers) definition.
      *
      * [MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl#locales_argument).
      *
-     * [Wikipedia](https://en.wikipedia.org/wiki/IETF_language_tag).
      */
-    type BCP47LanguageTag = string;
+    type UnicodeBCP47LocaleIdentifier = string;
 
     /**
      * Unit to use in the relative time internationalized message.
@@ -311,7 +310,7 @@ declare namespace Intl {
      * [Specification](https://tc39.es/ecma402/#table-relativetimeformat-resolvedoptions-properties)
      */
     interface ResolvedRelativeTimeFormatOptions {
-        locale: BCP47LanguageTag;
+        locale: UnicodeBCP47LocaleIdentifier;
         style: RelativeTimeFormatStyle;
         numeric: RelativeTimeFormatNumeric;
         numberingSystem: string;
@@ -454,7 +453,7 @@ declare namespace Intl {
          * [Specification](https://tc39.es/ecma402/#sec-intl-relativetimeformat-constructor).
          */
         new(
-            locales?: BCP47LanguageTag | BCP47LanguageTag[],
+            locales?: UnicodeBCP47LocaleIdentifier | UnicodeBCP47LocaleIdentifier[],
             options?: RelativeTimeFormatOptions,
         ): RelativeTimeFormat;
 
@@ -490,9 +489,9 @@ declare namespace Intl {
          * [Specification](https://tc39.es/ecma402/#sec-Intl.RelativeTimeFormat.supportedLocalesOf).
          */
         supportedLocalesOf(
-            locales: BCP47LanguageTag | BCP47LanguageTag[],
+            locales?: UnicodeBCP47LocaleIdentifier | UnicodeBCP47LocaleIdentifier[],
             options?: RelativeTimeFormatOptions,
-        ): BCP47LanguageTag[];
+        ): UnicodeBCP47LocaleIdentifier[];
     };
 
     interface NumberFormatOptions {