Extend the validateHighResolutionIconUrl function to check for valid image URL

[waldekmastykarz]

We should extend the _validateHighResolutionIconUrl function in the generated gulpfile.js to check if the specified URL is a valid image URL (extension ending with .png or .jpg). Currently we only check if the specified URL is an https URL. The problem with that is that if you provide a non-image URL (eg. https://contoso.com), your add-in will work as expected in OWA but won't show up in Outlook client which is almost impossible to track back to the URL specified in the HighResolutionIconUrl property in the manifest. If we could check for it, it would be very helpful for the developers.

[andrewconnell]

👍

[waldekmastykarz]

Shall I pick it up after we merge PR #111 into main to avoid merge issues given that a fix for this will modify the same files?

[andrewconnell]

Go for it

[waldekmastykarz]

👍