Refactor isMediaUrl to more generic API that supplies frontend metadata about media entities

Problem/Motivation

We added bespoke API for checking whether media entity is for an image (confusingly that's called isMediaUrl on the frontend). However, the frontend will need additional information about the media entity:

1. #3246365: [drupalMedia] Show the Image Media's default alt text that is being overridden

Proposed resolution

Refactor isMediaUrl to more generic API that supplies frontend metadata about media entities. The API should provide frontend the following information:

1. Whether the media entity contains an image
2. The media entity default alt text for the media if it contains an image

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet