refactor(parser): improve typings and do some refactoring (#305)

* dev: add response types

* dev: refactor `Parser#parseResponse()`

* dev: update YouTube parsers

* dev: update YouTube Music classes

* dev: update YouTube Kids classes

* dev: update core classes

* dev(Parser): fix some inconsistencies

* chore: update docs

* chore: update docs x2

* fix: export response types 

* chore(docs): update parser example

src/parser/helpers.ts

@@ -378,7 +378,7 @@ export type ObservedArray<T extends YTNode = YTNode> = Array<T> & {
     /**
      * Get the first item
      */
-    first: () => T | undefined;
+    first: () => T;
     /**
      * This is similar to filter but throws if there's a type mismatch.
      */