`SpanStyle` properties do not merge when appropriate

[JakeWharton]

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).

[JakeWharton]

With more advanced underlines coming in #628, we probably need to change this design anyway.

[JakeWharton]

Upstream bug: https://issuetracker.google.com/issues/397701274, not that we're blocked by it