feat(kanata): add HRM-optimized navigation layer

## HRM friendly S-tab/tab

S-tab and tab stay on the homerow while playing nice with Ctrl and Alt mods.

## Add cross-platform quit shortcut on HRM nav layer

@qut → [Q]
- @qut allows closing apps on any OS (C-q on Linux, A-f4 on Windows, etc.), optimizing muscular memory
- it is more coherent to map @qut on [Q]

@pad → [A]
- @pad thus mirrors @fun on the right hand
- allows using the numpad’s arrow keys without locking the layer (layer-while-held)

## Map esc/del on [A]/[V]

The goal is to allow escaping and deleting using the left hand only.

## Media controls in funpad

Because funpad stands for fun!
Rationale: mod keys in the existing funpad are pointless with HRMs.

README.md

@@ -157,6 +157,22 @@ without holding the key until escaped with <kbd>Alt</kbd> or <kbd>AltGr</kbd>.
   <em>Fn layer toggled</em>
 </p>
 
+##### HRM friendly variant
+
+The basic `Navigation` layer’s `WASZXCV` shortcuts are useful for some layouts, but they can be seen as a waste of layer space when using Home Row Mods.
+
+This variant replaces QWERTY shorcuts by the following:
+- cross-platform quit shortcut
+- HRM optimized `tab` and `S-tab`
+- easy to use `escape` and `delete`
+- three empty spots for your own keys or layers
+
+![HRM friendly Vim navigation layer on a 33-key keyboard](./img/vim_hrm_navigation.svg)
+
+Furthermore, the `Fn` layer is modified to have media controls instead of modifiers:
+
+![HRM friendly Fn layer on a 33-key keyboard](./img/fn_hrm.svg)
+
 ### 5. Keyboard layout
 
 Choose your keyboard layout among the available ones for Arsenik to work