Intl.RelativeTimeFormat does not correctly reflect the spec

[longlho]

TypeScript Version:
3.9
Search Terms:
Intl.RelativeTimeFormat
Code

Expected behavior:

Actual behavior:
Reading through the lib2020.intl.d.ts I noticed a couple of issues in the declarations:

1. Locale should not be BCP47LanguageTag, it should be UnicodeBCP47LocaleIdentifier.
2. RelativeTimeFormat interface and const declaration should be combined into a single class.

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