Vendor relative-time-element as local web component (#36853)

Replace the `@github/relative-time-element` npm dependency with a
vendored, simplified implementation.

- Support 24h format rendering [PR
329](https://github.com/github/relative-time-element/pull/329)
- Enable `::selection` styling in Firefox [PR
341](https://github.com/github/relative-time-element/pull/341)
- Remove timezone from tooltips (It's always local timezone)
- Clean up previous `title` workaround in tippy
- Remove unused features
- Use native `Intl.DurationFormat` with fallback for older browsers,
remove dead polyfill
- Add MIT license header to vendored file
- Add unit tests
- Add dedicated devtest page for all component variants

---------

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: Claude claude-opus-4-6 20250630 <noreply@anthropic.com>

web_src/js/webcomponents/polyfills.ts

@@ -1,21 +1,3 @@
-try {
-  // some browsers like PaleMoon don't have full support for Intl.NumberFormat, so do the minimum polyfill to support "relative-time-element"
-  // https://repo.palemoon.org/MoonchildProductions/UXP/issues/2289
-  new Intl.NumberFormat('en', {style: 'unit', unit: 'minute'}).format(1);
-} catch {
-  const intlNumberFormat = Intl.NumberFormat;
-  Intl.NumberFormat = function(locales: string | string[], options: Intl.NumberFormatOptions) {
-    if (options.style === 'unit') {
-      return {
-        format(value: number | bigint | string) {
-          return ` ${value} ${options.unit}`;
-        },
-      } as Intl.NumberFormat;
-    }
-    return intlNumberFormat(locales, options);
-  } as unknown as typeof Intl.NumberFormat;
-}
-
 export function weakRefClass() {
   const weakMap = new WeakMap();
   return class {