Fix raw attribute sources with multiple nodes

[alecgeatches]

Description

See #81 for a description of the problem. In short, if a block with a raw source like core/html has multiple top-level nodes, the Block Data API returns only the HTML of the first node.

Block HTML:

<!-- wp:html -->
<h1>First node</h1><h2>Second node</h2>
<!-- /wp:html -->

Incorrect output:

{
    "name": "core/html",
    "attributes": {
        "content": "<h1>First node</h1>"
    }
}

The source_block_raw function responsible for gathering this HTML has been changed to use the parent wrapper block's inner HTML. This will collect all node HTML as well as preserving inner whitespace in the HTML content.

Steps to Test

1. Check out PR.
2. Run wp-env start and composer install.
3. Run composer run test
4. See successful tests.

[alecgeatches]

Merging! Test failures are from block bindings differences in latest WordPress, unrelated to the changes here.