Add confirm modal stories to vue shared

What does this MR do and why?

In !71395 (merged) we introduced a VueJS equivalent of the confirm danger modal, the modal was included in the vue_shared directory as it will be re-used in multiple locations as part of

This MR adds storybook stories to document some basic usage of this new modal. Some small visual changes are introduced:

• replacing the text-danger class used to highlight the warning message, instead adding a GlAlert
• correctly applying the form-control class to the form input instead of the form group

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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