Keymap: Spaceman Spiff layout for KBD75 (#3595)

Keymap: Spaceman Spiff layout for KBD75 (#3595)

![Keyboard Layout](https://i.imgur.com/tQLYDvu.png)

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Designed specifically to make switching back-n-forth with the Apple's MacBook Pro keyboard intuitive. Has a slight tendency toward readline/vim keybindings.

Caps locks becomes an `Esc` when pressed alone, or a `Ctrl` when pressed with another key.

Base Layer
----------

As simalar to the Apple keyboard as possible. Notiable exception is `Caps Lock`:
- `Esc` when pressed alone
- `Ctrl` when pressed with another key

Control+ Layer
--------------

Left `Ctrl` key switches to the "Control+ Layer". This layer mostly acts like a control key in most cases, with a few exceptions:

- `Ctl+` + `hjkl` are vim-style motion keys
- `Ctl+` + `p` and `Ctl+` + `n` are page up and down
- `Ctl+` + `Backspace` is forward delete

Fn Layer
--------

Audio Controls:
- `Fn` + `a` Volume Down
- `Fn` + `s` Volume Up
- `Fn` + `d` Mute

To flash this layout you need to press `Fn+Backspace`