Childless alternatives of CodeBlock, Heading can be removed

[halilozercan]

CodeBlock and Heading have alternative overloads that take literal text content. These alternatives are proxies to actual implementation which takes composable children. Only child that is passed to actual composable is a Text(...).

This API style of having alternatives is no longer common in Compose, e.g. Button does not offer a label alternative. We should stick with slot API most of the time.