Docs: add step for build and asset file

[tharsheblows]

There was a missing step in the plain javascript documentation which missed out adding the asset file. Issue #40447

Done during WCEU 2022 Contributor day! 🎉

[github-actions]

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[tharsheblows]

For ticket Blocks without asset.php file cannot be registered

[ryanwelcher]

LGTM! Thanks for putting this together!

[github-actions]

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[chairmanbrando]

I'm not sure if this is the correct spot for this comment, but I landed here via Google, so here it goes...

The block.json docs are still confusing, IMO. I'm not sure I would've figured out why my block wasn't loading its JS file if I hadn't thought to check the WP_DEBUG_LOG file.

PHP Notice: Function register_block_script_handle was called incorrectly. The asset file ([...]/stock-ticker.asset.php) for the "script" defined in "dp/stock-ticker" block definition is missing.

The type definitions for both style and script are the same in the docs, WPDefinedAsset|WPDefinedAsset[], yet they don't fully act the same. Styles can be registered and loaded without a matching .asset.php file while scripts cannot. In fact, core's register_block_style_handle() seems to show that .asset.php isn't even looked for.

It makes sense; scripts loaded without their dependencies will break things. Yet despite showing "file:./script.js" in the example, it's not stated that your script will not be loaded without a matching .asset.php -- at least when using the "file:./[...]" method.

It was hard to find useful info via Google, so if I hadn't had debugging enabled to alert me to the problem, I likely would've soon set up my own block scripts loader in wp_enqueue_scripts using has_block(). D: