Fix token rendering in @vue/compat dev builds

What does this MR do?

Fix GlTokenSelector in @vue/compat dev builds

This fixes various rendering issues in the GlTokenSelector component under development builds of @vue/compat.

There is a bug in @vue/compat such that, in a development build, the presence of a comment node surrounded by whitespace is considered valid slot content, which prevents slot default content from rendering.

This only happens in development builds of @vue/compat. Production builds strip out template comments by default.

Vue 2 and 3 proper do not have this bug, in either development or production builds. See the issue linked below for more details.

Addresses #2950

Add visual tests for GlTokenSelector

The component has various accessibility issues which the axe tooling reveals during screenshot tests, so we add aria-allowed-attr to the skip list.

See #2215 (closed).

Screenshots or screen recordings

Note: These changes are only visible in a development build of Storybook.

Before	After
simplescreenrecorder-2025-08-14_09.48.39	simplescreenrecorder-2025-08-14_09.49.17

Integrations

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Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
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• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.