fix(template-require-media-caption): match kind="captions" case-insensitively

HTML enumerated attribute values are case-insensitive per the spec:
https://html.spec.whatwg.org/multipage/common-microsyntaxes.html#enumerated-attributes

Before: <track kind="Captions"> and kind="CAPTIONS" were treated as
missing a captions track.

Lowercase the value before comparison. Matches jsx-a11y and vue-a11y
(both lowercase before comparing).

Author: johanrd

lib/rules/template-require-media-caption.js

@@ -74,7 +74,7 @@ module.exports = {
           }
 
           if (kindAttr.value?.type === 'GlimmerTextNode') {
-            return kindAttr.value.chars === 'captions';
+            return kindAttr.value.chars.toLowerCase() === 'captions';
           }
 
           return false;

tests/lib/rules/template-require-media-caption.js

@@ -31,6 +31,10 @@ ruleTester.run('template-require-media-caption', rule, {
     </template>`,
 
     '<template><video><track kind="captions" /></video></template>',
+    // HTML enumerated attribute values are case-insensitive, so "Captions" is
+    // the same as "captions" for the track element. Matches jsx-a11y/vue-a11y.
+    '<template><video><track kind="Captions" /></video></template>',
+    '<template><video><track kind="CAPTIONS" /></video></template>',
     '<template><audio muted="true"></audio></template>',
     '<template><video muted></video></template>',
     '<template><audio muted={{this.muted}}></audio></template>',