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/classes/SectionList.ts

@@ -7,8 +7,8 @@ class SectionList extends YTNode {
   target_id?: string;
   contents;
   continuation?: string;
-  header;
-  sub_menu;
+  header?;
+  sub_menu?;
 
   constructor(data: any) {
     super();