Add a way to supply a fallback image to the Image component

[fabiankaegy]

Description of the Change

This is an exploration of how we could potentially handle adding a fallback image option to the image component.

Closes #161

How to test the Change

Changelog Entry

Added - Ability to provide a fallback image for the Image component

Credits

Props @ncoetzer @sudar @ajmaurya99

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[ncoetzer]

@fabiankaegy Essentially, I think we could have gone two ways about this:

1. The way you approached it here by using a single prop structured as an object with two values, i.e., id & fallback, respectively. However, I'm just wondering about any possible confusion about what type of data needs to be provided for these two values, i.e., number vs. string.
2. OR perhaps it would be more clear (and less complex) to define the fallback image URL as a separate prop (optional), e.g.,

"imageId": {
    "type": "number",
    "default": null
},
"fallbackImageUrl": {
    "type": "string",
    "default": ""
}