compiler: Have a `key-sequence` type

[hunger]

... which is lowered to a string for now.

Add a new type keyboard-shortcut, that is parsed from @keys(shift + control + a) and gets lowered to a string in Rust and C++ and the interpreter for now.

[hunger]

I wanted to keep this unmerged for the time being and build on top of this PR.

[hunger]

Newest version now accepts keys from the Keys namespace. To match up with what is defined in there, I also renamed the modifiers to start with uppercase characters. I have ignored the right-side variant of the modifier keys for now.

[hunger]

A bit of progress was made:

• Old branch: Rebased the entire thing on top of current master
• New commit: Printing was fixed to match inputs better (should be squashed)
• New commit: Add a KeyboardShortcut struct to the builtin structs and expose that to Slint
  • Rust works
  • C++ still converts to a string somewhere and fails, should be easy to fix though
  • Python and JS is untested so far