Switch to pointer events and remove custom event handling logic

[maribethb]

Is your feature request related to a problem? Please describe.

Currently Blockly has complex code that tries to unify mouse and touch events. This is all custom code that predates the invention and subsequent adoption of PointerEvent by all major browsers. The custom code is complex and a source of bugs e.g. #6508

Describe the solution you'd like

Remove the custom code and switch to PointerEvents.

Describe alternatives you've considered

Additional context

Assigning to Aaron according to Q4 OKRs, please feel free to reassign if this changes.

[rachel-fenichel]

Internal doc with project description: https://docs.google.com/document/d/1vSlRNKOiS82jyqJr1VgEYcGwKEeUcqxPMS_YVoHZk60/edit?usp=drive_open&ouid=106800571011058844251