Add fields for alternative text labels to settings form

Lucked out that I just took over this module and was triaging the backlog. Should be able to update the labels.

Hi @smustgrave, I've tested shared MR- MR !27 mergeable on Drupal version- 10.3-dev. The patch was applied successfully and looks good to me.

Testing Steps:

• Add/download Module "External Links".
• Go to the shared path- /admin/config/user-interface/extlink and check, Added Before patch results.
• Download this share MR and Apply.
• Go back to- /admin/config/user-interface/extlink & Re-verify this

Testing Results:
After applying the patch, I was able to update the labels. Added after result. RTBC++

Thank you for the quick fix! The diff also applies and works great in v8.x-1.7.