`SpanStyle` properties do not merge when appropriate

```kotlin
buildAnnotatedString {
	append("one")
	withStyle(SpanStyle(textStyle = Underline)) {
		append("two")
		append(buildAnnotatedString {
			withStyle(SpanStyle(textStyle = Italic)) {
				append("three")
			}
		})
	}
	append("four")
}
```

This is also a problem in Compose UI with its `TextDecoration` (e.g., underline + strikethrough).