Log error when `compose`-ing on a pseudo element.

CSS Blocks should throw an error when trying to use `compose` in a pseudo element.

```css
/* Src Block */
.other-thing { color: red }
/* In another file */
.my-class::before {
composes: "other-thing";
}
```

This doesn't make sense since we will try to apply a class to a pseudo element – which can not receive classes.

Alternative: Inject a new selector that will apply the same code block for pseudos. For example, the above could output:
```css
.other-thing, .my-class::before { color: red; }
```

The issue names no source file, test, or entry point; begin by tracing CSS Blocks handling of `composes` with pseudo-element selectors. Resolve whether the expected outcome is an error or selector injection, then add coverage for the shown example and verify the chosen behavior.

Written by the indexing model from the issue text.