VR keyboard F-keys: F1-F12 in the suggestion strip while AltGr is active

keyboard.vr.functionKeys.enable (default off): while AltGr (Fn on layouts
without AltGr) is one-shot, locked or held, the strip shows F1-F12 instead
of the suggestions. A tap types the extra key VKX_F<n>, which the extra
keys' patch presses with xdotool together with the active Ctrl/Alt/Shift
(Alt+F4, Ctrl+F5) and releases the toggles; the text model resets, like
for Esc. The suggestions are kept as they are meanwhile, so when AltGr goes
off the strip shows the same items and selection again.

- vr-keyboard/function-keys.js: what the strip shows, the F-key keys.
- patch.js (VERSION 27): the strip's view, F-key picks, AltGr changes via
  a componentDidUpdate on the keyboard instance (and the poll).
- Allowlist: F1-F12 with any modifiers; Enter still refused.
- relay.mjs: up to 12 strip items.
- Assertions: needs keyboard.vr.enable, extraKeys.enable and a strip
  position "above"/"below" ("inside" would hide the AltGr number row).
- Tests: function-keys.test.mjs (switching and restoring, keys against
  the allowlist, text model), allowlist F-key cases.

docs/development.md

@@ -105,12 +105,14 @@ modules/
     textmodel.js               Steam (argument of patch.js): what the keyboard typed
     corrector.js               Steam (argument of patch.js): corrections, completions
     gesture-input.js           Steam (argument of patch.js): a gesture's own events, its hand's bridge path
+    function-keys.js           Steam (argument of patch.js): F1-F12 in the strip while AltGr is active
     suggestions-panel/
       patch.js, unpatch.js     SteamVR: the strip as a panel above/below the keyboard
       relay.mjs                service vr-keyboard-relay: strip state Steam <-> SteamVR
     dictionary.nix, gen-dict.py  build: dictionary from wordfreq + Hunspell
     check.nix, tests/          test (also built before the patch): text model,
-                               corrector, swipe-decoder accuracy, gesture input
+                               corrector, swipe-decoder accuracy, gesture input,
+                               the F-key strip
   vr-keyboard-controllers.nix  controller bridge for keyboard features (internal option)
   vr-keyboard-controllers/
     bridge-patch.js, unpatch.js  SteamVR: keyboard pose, tips, laser hits, triggers