[node] add base64url BufferEncoding option

New encoding option was introduced in nodejs/node#36952 and 
is available since v15.7.0. It is documented on the website: 
https://nodejs.org/api/buffer.html#buffer_buffers_and_character_encodings

types/node/globals.d.ts

@@ -71,7 +71,7 @@ declare var module: NodeModule;
 declare var exports: any;
 
 // Buffer class
-type BufferEncoding = "ascii" | "utf8" | "utf-8" | "utf16le" | "ucs2" | "ucs-2" | "base64" | "latin1" | "binary" | "hex";
+type BufferEncoding = "ascii" | "utf8" | "utf-8" | "utf16le" | "ucs2" | "ucs-2" | "base64" | "base64url" | "latin1" | "binary" | "hex";
 
 type WithImplicitCoercion<T> = T | { valueOf(): T };