feat(camera): allow for additional accept types for image selection

[sean-perkins]

Resolves #80

Adds an additional property to the pwa-camera to allow developers to specify additional acceptable file types when a camera device is not available. This feature is intended to be used to restrict the file types to more specific patterns than the default image/* matches to. For example, restricting to only png or jpeg image file types.

[Flusinerd]

Hi, I just stumbled across this, since I was looking for this functionality.
Ty first of all for the implementation.

When will this be merged?

[jcesarmobile]

This needs to be added to camera-modal and camera-modal-instance too.

The normal usage for Camera plugin and most pwa-elements users is to create a camera-modal, camera-modal then creates the camera-modal-instance, and camera-modal-instance uses the pwa-camera internally, users don't usually use the pwa-camera directly. So for being able to set the accept in the pwa-camera, all intermediary accept props are needed.

Also, #80 is about the accept attribute of file picker shown if the user clicks the pick-image button, this has been applied to the fallback picker when the camera is not available, they are different.
I think it should be applied to both pickers (and update the docs accordingly since now it only talks about the fallback).