Add font group protocol

[LiYanan2004]

Add MarkdownFontGroup protocol for better experience when you want to modify multiple types of font.

Deprecation

• The old MarkdownFontProvider has been deprecated.
• markdownFont(_:) modifier has been deprecated.

If you are using MarkdownFontProvider, you should migrate to MarkdownFontGroup.

Example

Define your font group:

struct MyCustomFontGroup: MarkdownFontGroup {
    var h1: Font { Font.largeTitle.weight(.black) }
}
extension MarkdownFontGroup where Self == MyCustomFontGroup {
    static var custom: MyCustomFontGroup { .init() }
}

Attach the font group:

MarkdownView(...)
    .fontGroup(.custom)