Migrate Keyboard APIs to async

[inancgumus]

Tasks

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1. Prepare Keyboard for async migration #1313
   mapping refactor stability +3
   
2. Async Keyboard #1314
   async +1
   
3. Update to Async Keyboard k6-docs#1592
   Area: browser +1
   
4. Create documentation for the Keyboard class #765
   documentation good first issue hacktoberfest +3
   
5. Update Mouse and Keyboard interface to return Promises k6-DefinitelyTyped#45
   browser +1
   
6. Add release notes

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What

Migrate the following APIs according to the How section in #428:

Class	Method	Current k6 Return	PW return
Keyboard	down	void	Promise
Keyboard	insertText	void	Promise
Keyboard	press	void	Promise
Keyboard	type	void	Promise
Keyboard	up	void	Promise

For details on the Why and How, check the overarching issue in #428.