Verify when a label is not a valid Maki icon (static marker annotation url)

[Guardiola31337]

Follow up from #479

Static marker annotation url label options are:

• 1 alphabet letter [a-zA-Z]
• A number from 0 to 99
• A valid Maki icon

At the moment, valid Maki icon option is not checked.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically detected as stale because it has not had recent activity and will be archived. Thank you for your contributions.