Use Keyboard API to lock screen for supported browsers

When using fullscreen mode, hitting the escape key will exit out of
fullscreen mode.  To stay in fullscreen mode but pass the escape
key through, we can use the Keyboard API to require the escape key
be held down https://wicg.github.io/keyboard-lock/#escape-key.

Tested in Chrome which supports Keyboard API and Firefox which does
not https://caniuse.com/?search=navigator.keyboard with no errors
in either case.

client/src/components/video.vue

@@ -194,7 +194,7 @@
 <script lang="ts">
   import { Component, Ref, Watch, Vue, Prop } from 'vue-property-decorator'
   import ResizeObserver from 'resize-observer-polyfill'
-  import { elementRequestFullscreen, onFullscreenChange, isFullscreen } from '~/utils'
+  import { elementRequestFullscreen, onFullscreenChange, isFullscreen, lockKeyboard, unlockKeyboard } from '~/utils'
 
   import Emote from './emote.vue'
   import Resolution from './resolution.vue'
@@ -417,6 +417,7 @@
 
       onFullscreenChange(this._player, () => {
         this.fullscreen = isFullscreen()
+        this.fullscreen ? lockKeyboard() : unlockKeyboard()
         this.onResize()
       })