Support user theme preference when constructing favicons dynamically by allowing the media metadata field

[jimCresswell]

Goals

Enable the dynamic construction of link elements like like <link rel="icon" href="/icon-512-dark?abc123" media="(prefers-color-scheme: dark)"> from app/icon.tsx files, so that the browser can choose appropriate favicon images depending on user preference. At the moment the media metadata is stripped out when calling generateImageData, which makes sense, but in this respect a favicon has additional metadata to a standard image.

Non-Goals

1. Supporting media metadata for images in general, just for favicons

Background

It's needed for accessibility, and to support a standard browser mechanism that people already use.

Proposal

Create a new metadata subclass generateIconMetadata that supports all of the link, rel=icon metadata fields.

I can contribute.