Add API to configure the fallback fonts provider when all default met…
…hods return null (#935)
New API:
```kotlin
fun TypefaceFontProvider.createAsFallbackProvider(): TypefaceFontProvider
fun FontMgr.defaultWithFallbackFontProvider(fallback: TypefaceFontProvider): FontMgr
```
Usage:
```kotlin
val fontCollection = FontCollection()
val notoEmojiTypeface = Typeface.makeFromData(Data.makeFromBytes(notoEmojisBytes))
val fallbackProvider = TypefaceFontProvider.createAsFallbackProvider().apply {
registerTypeface(notoEmojiTypeface)
}
fontCollection.setDefaultFontManager(FontMgr.defaultWithFallbackFontProvider(fallbackProvider))
```
<img width="400" alt="Screenshot 2024-06-07 at 14 48 28"
src="https://github.com/JetBrains/skiko/assets/7372778/76dc1330-d8e2-4be7-ad92-68214b058b4f">
