feat: make double tap to hold layer optional per key (#662)

* feat: make double tap to hold layer optional per key

* test: fix test serializer

* fix: remove "application start" text

* Add double-tap.svg

* Add closing dot at the end of the sentence.

* fead: add double-tap icon

* Bundle firmware version 8.3.0

* feat: 'layer-double-tap' feature flag

* feat: convert SwitchLayerMode to string enum

packages/uhk-web/src/svgs/keyboard/icons/kbd__fn--double-tap.svg

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg"
+     xmlns:xlink="http://www.w3.org/1999/xlink"
+     viewBox="0 0 26 26"
+     version="1.1"
+     width="26"
+     height="26">
+    <g id="surface1">
+        <path style=" "
+              d="M 13 4 C 8.5625 4 4.667969 6.410156 2.59375 10 C 2.316406 10.484375 2.484375 11.097656 2.96875 11.375 C 3.453125 11.652344 4.066406 11.484375 4.34375 11 C 6.070313 8.011719 9.289063 6 13 6 C 16.710938 6 19.929688 8.007813 21.65625 11 C 21.933594 11.484375 22.546875 11.652344 23.03125 11.375 C 23.515625 11.097656 23.683594 10.484375 23.40625 10 C 21.332031 6.410156 17.4375 4 13 4 Z M 13 8 C 8.59375 8 5 11.59375 5 16 C 4.996094 16.359375 5.183594 16.695313 5.496094 16.878906 C 5.808594 17.058594 6.191406 17.058594 6.503906 16.878906 C 6.816406 16.695313 7.003906 16.359375 7 16 C 7 12.675781 9.675781 10 13 10 C 16.324219 10 19 12.675781 19 16 C 18.996094 16.359375 19.183594 16.695313 19.496094 16.878906 C 19.808594 17.058594 20.191406 17.058594 20.503906 16.878906 C 20.816406 16.695313 21.003906 16.359375 21 16 C 21 11.59375 17.40625 8 13 8 Z M 13 12 C 10.789063 12 9 13.789063 9 16 C 9 18.210938 10.789063 20 13 20 C 15.210938 20 17 18.210938 17 16 C 17 13.789063 15.210938 12 13 12 Z "/>
+    </g>
+</svg>