Create the skeleton for a keyboard shortcut system

[commonspider]

This PR implements a keyboard shortcuts system based on the structure of the clipboard system.
Different modes can be registered by the handler. The first enabled one, in order of priority, will handle keydown events.

Technical details

• shortcuts.ts: holds the store and interface.
• AppContext.svelte: initializes shortcutsHandler and sets the absolute priority of modes with a list of names.
• TableView.svelte: instantiates SelectMode and EditMode and passes them to the Sheet.
• Sheet.svelte: registers the two modes and enables SelectMode, only when it is focused.
• CellWrapper.svelte: enables EditMode only when itself is in edit mode.
• AppShortcutsHandler.ts: keeps tracks of modes and calls the correct one on keydown.
• EditMode.ts: currently empty.
• SelectMode.ts: calls the "Set to NULL" shortcut.
• setNull.ts: implements the shortcut.

What's left

• All the requirements in Add keyboard shortcuts for setting a cell to NULL #936
• To use the same code as the context menu's setNull.ts
• To make the system more robust and easier to use

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the develop branch of the repository
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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[zackkrida]

@commonspider would it be helpful for someone (likely @seancolsen) to review what you've done here so far?

[commonspider]

@zackkrida Yes thank you. @seancolsen I've followed the structure of the clipboard system and changed it a little bit according to your description on Matrix. If my attempt is acceptable, I can do some tests and push a final version.
Also, should I use the function in your file setNull.ts as it is, or do I keep on implementing my own?

[pavish]

Looks like GH automatically closed this PR since the context_menu branch was merged into develop (since merging a PR deletes the branch by default). The target branch should have automatically changed to develop but I'm not sure what went wrong.

@commonspider Could you please open a new PR with develop as the target branch?