chore(vue3): use more reliable non-text vnodes detection

What does this MR do?

Vue3 uses symbols as tags for text nodes. We can check if vnode is a tag by checking if it is string

This is forward-compatible change to support @vue/compat #1981 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• [-] If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• [-] If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• [-] If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • [-] GitLab: mr_url
  • [-] CustomersDot: mr_url
  • [-] Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.