refactor(dark): use palettes through url queries in test pages (#29238)

Issue number: N/A

---------

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## What is the current behavior?
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If a dev wants to view a test page in dark mode, they have to manually
add the styles. This can lead to a slowdown. Plus they can't use
Playwright's `goto` to test both light and dark. In order to test dark
mode with Playwright, the dev would need to use `setContent` instead of
`goto`.

## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->

Dark mode can be added to any page by appending `palette=dark` to the
URL.

- The param will be used to add a link tag with the correct palette
file.
- Playwright will load the correct palette file when a dev uses `goto`
and `{ themes: ['dark'] }`

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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screenshots of how the component looks before and after the change. -->

I recommend using badge to try this out. It already has a `goto` in the
basic tests.

core/scripts/testing/scripts.js

@@ -14,6 +14,20 @@
     document.head.appendChild(style);
   }
 
+  /**
+   * The term `palette` is used to as a param to match the
+   * Ionic docs, plus here is already a `ionic:theme` query being
+   * used for `md`, `ios`, and `ionic` themes.
+   */
+  const palette = window.location.search.match(/palette=([a-z]+)/);
+  if (palette && palette[1] !== 'light') {
+    const linkTag = document.createElement('link');
+    linkTag.setAttribute('rel', 'stylesheet');
+    linkTag.setAttribute('type', 'text/css');
+    linkTag.setAttribute('href', `/css/palettes/${palette[1]}.always.css`);
+    document.head.appendChild(linkTag);
+  }
+
   window.Ionic = window.Ionic || {};
   window.Ionic.config = window.Ionic.config || {};