Feat: Multiple Inheritance

Hi 👋!

I think this project is really interesting and I was wondering, why is it that I can only extend a block once? I'm in a situation where I want to import 2 blocks.

```
@block-reference buttons from "../../styles/blocks/buttons.block.css";
@block-reference loading from "../../styles/blocks/loading.block.css";

:scope {
extends: buttons;
extends: loading;
}
...

Generates Error:

{ Error: [css-blocks] BlockSyntaxError: A block can only be extended once. (tmp/broccoli_css_blocks_analyze-output_path-Co5Mjhle.tmp/src/ui/components/.../stylesheet.css:6:3)
```

I think the guide may be a bit confusing because I just want to use classes from 2 different files in one file and I really don't know any way of doing that.

Here's my package.json (the relevant parts [I think?]):
```
"@css-blocks/ember-cli": "^0.20.0-beta.3",
"@glimmer/application": "^0.9.1",
```

The issue names no files or tests. Start by locating the parser or validation path that emits “A block can only be extended once,” then trace how @block-reference and extends are represented. Done means the requested multiple inheritance behavior is defined and implemented with coverage for extending a block from both referenced files.

Written by the indexing model from the issue text.