Forms: Add asset pipeline and style support for custom fields

[enejb]

Fixes #

Proposed changes

Adds style support and convention-over-configuration asset discovery to the custom form field registry:

• Style support in field registry - Add editor_style, view_style, and dashboard_style registration alongside existing script support, with corresponding _deps and _ver options for each.
• Convention-over-configuration asset discovery - When plugin_file is provided, assets are automatically resolved from conventional paths (build/index.js, build/editor-style.css, build/view.js, build/view-style.css, src/dashboard.js, build/dashboard-style.css). Dependencies and versions are loaded from build/index.asset.php. Individual assets can be overridden or disabled (set to false).
• Editor styles via register_block_type - Editor stylesheets are registered through register_block_type() block args (editor_style) rather than direct wp_enqueue_style, ensuring styles load correctly inside the block editor iframe.
• View and dashboard style enqueuing - View styles are enqueued during field rendering; dashboard styles are enqueued alongside dashboard scripts.
• Docs update - Rewrites custom-fields.md into a template-driven format with LLM prompt template, file templates for all field components, webpack configuration, build instructions, and an emoji rating worked example.

Other information

This is PR 4 of 5 in a stacked PR series adding custom field extensibility to Jetpack Forms:

PR	Focus
#47931	PR 1: PHP foundation (registry, rendering, validation)
#47932	PR 2: JS/Editor API (block registration, editor integration)
#47933	PR 3: Label block, dashboard integration, Interactivity API
#47930	PR 4: Asset pipeline & style support (this PR)
#47929	PR 5: Documentation & changelog

• Generate changelog entries for this PR (using AI).

Related product discussion/links

• Part of the Jetpack Forms custom fields extensibility project.

Does this pull request change what data or activity we track or use?

No changes to tracked data or activity.

Testing instructions

1. Style registration defaults - Confirm the registry now accepts editor_style, view_style, and dashboard_style arguments with corresponding _deps and _ver options.

2. Convention-over-configuration asset resolution - Create a test custom field plugin with plugin_file => __FILE__ and place files at the conventional paths (build/index.js, build/editor-style.css, build/view-style.css). Verify the registry auto-discovers and registers them without explicit URL configuration. Verify setting an asset to false disables it.

3. Editor styles in iframe - Register a custom field with an editor_style. Open the block editor and confirm the stylesheet loads inside the editor iframe (inspect the iframe document for the <link> tag with handle jetpack-forms-field-{type}-editor).

4. View style enqueuing - Add a custom field with a view_style to a form. View the form on the frontend and confirm the stylesheet is enqueued (check page source for the style handle jetpack-forms-field-{type}-view).

5. Dashboard style enqueuing - Register a custom field with a dashboard_style. Open the Jetpack Forms responses page and confirm the stylesheet is loaded.

6. Explicit override - Register a field with both plugin_file and an explicit editor_script URL. Verify the explicit URL takes precedence over convention-based resolution.

7. Asset file loading - Place a build/index.asset.php in the plugin directory returning custom dependencies and version. Verify these are used for the editor script registration instead of defaults.

[github-actions]

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• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (Jetpack), and enable the add/form-fields-asset-pipeline branch.
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Code Coverage Summary

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