Add additional shell, buffer, and application command types

[ulyssa]

This PR adds new CommandType::Shell, CommandType::Buffer, and CommandType::Application variants, which allow for additional command bar uses, primarily with the Kakoune, Emacs, and consumer application keybindings respectively.

I've used enum-map to create a new CommandMap type to make it easier for consumers to instantiate array-like mappings that contain already-initialized separate entries for every CommandType, which I found helpful for simplifying the logic in both RegisterStore and in scansion.

I haven't updated the Kakoune or Emacs keybindings yet to take advantage of Shell and Buffer respectively. They'll both need new actions first that consume the register values.

resolves #69
resolves #130