feat(textarea): Add debounce/lazy support

What does this MR do?

feat(textarea): Add debounce/lazy support

This is very similar work to that found in #631 (closed), but applied to GlFormTextarea rather than GlFormInput.

There's an integration MR for GitLab, just to make sure this doesn't break anything. It shouldn't! If so, that integration MR can simply be 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: gitlab!39599 (closed)
  • [-] Customers Portal: mr_url
  • [-] Status Page: mr_url
• [-] Added the ~"component:*" label(s) if applicable.