Add a Hotkey Manager

[BroJac5246]

The Problem

Scratch Addons now has a ton of addons that use hotkeys/keyboard shortcuts/etc. This is great! You can perform tons of neat little actions. However, this comes with two major problems:

1. There are now way too many hotkeys to keep track of– and when you throw in some of Scratch's lesser known ones, it can be very confusing for both users and developers.
2. The wide range of devices & browsers can cause compatibility issues. See the list of related issues below.

In #4662 (also linked below), WorldLanguages and I came to the conclusion that a hotkey section in the settings would be the best solution. However, the specifics need to be discussed, which is part of why I'm opening this.

Here's what we need to consider:

1. Where would the hotkeys settings show up? In both the full screen settings and the popup, or just the full screen settings? Because of how important this will be, I think we should go with the former, but that's just my opinion.
2. How will the new approach to hotkeys be? This is a HUGE question. We already know there will be a dedicated manager, but how will this work? How will hotkeys be managed? Will addons specify in their addon.json that they would like to request a hotkey(s)?
3. Will the hotkey manager allow users to change the hotkeys included in vanilla scratch? This comment by WL demonstrates how this could be useful. (Read my comment after that one as well.) This could be complicated, though, which we need to consider.
4. Will different hotkeys be suggested, or even allowed, based on OS, to help users fix compatibility issues before they even arise?
5. Will additional hotkeys be introduced when this is implemented, such as those recommended in Keyboard Shortcuts for navigating the Scratch website #4725
6. What will happen to addons that use hotkeys? Will these be hidden from users? What if they need to have additional settings?
7. How will migration work? This was somewhat mentioned in Points 2 and 6, but what will happen to existing addons? Will they need to be modified for the new approach? Will we have to add a hotkey option to the addon manifest (addon.json)? How will users' settings be ported?
8. Not a big problem, but we also might need to remember to implement credits into the hotkey manager, depending on the approach we take.

Describe alternatives you've considered

Not implement a hotkey manager and let users suffer?

Issues that Demonstrate the Need for this

Issues that demonstrate compatibility issues:
#4466 and #4662

Issues that suggest keyboard shortcuts:
#4725 and #4888 (Plus probably a ton more that I'm too lazy to find)

[WorldLanguages]

Thanks for merging the main points of discussion into this single issue!

[RedGuy12]

Related: ScratchAddons/manifest-schema#24

[WorldLanguages]

Discussion is happening on a different issue (#5145), but the list of points to consider is still relevant.