Build: Fix import map ordering in wp-build page template

[lezama]

Description

The page-wp-admin.php template generated by @wordpress/build uses import("@wordpress/boot") inside a classic inline script (wp_add_inline_script). This is incorrect: import() with a bare specifier requires the import map to already be in the DOM.

In admin_print_footer_scripts, both _wp_footer_scripts and print_import_map are registered at priority 10, but _wp_footer_scripts comes first in registration order. The classic inline script therefore always appears before the import map in the HTML output.

This works by accident in most environments — browsers typically defer the specifier resolution to a microtask, by which point the import map has been parsed. But any plugin that changes execution timing (e.g. converting scripts to `type="module"` via `script_loader_tag`) breaks it:

```
TypeError: Failed to resolve module specifier '@wordpress/boot'
```

This affects all wp-build pages: Forms, Font Library, Connectors.

Fix

Two changes:

1. Template (page-wp-admin.php.template): The inline script no longer calls import(). Instead it sets the boot config as a plain global (window.__wpBuildBootConfig).

2. Loader (build.mjs): The previously empty loader.js stub now contains the actual boot code — it imports @wordpress/boot and calls initSinglePage with the config from the global. Since loader.js is a registered script module, it executes after the import map by definition.

Testing

1. Build a project that uses @wordpress/build with a wp-admin page layout
2. Install The Events Calendar
3. Navigate to the wp-build page — should render normally
4. Check console — no module specifier errors
5. With Gutenberg plugin active, also test /wp-admin/themes.php?page=font-library-wp-admin and /wp-admin/options-general.php?page=options-connectors-wp-admin

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Co-authored-by: lezama <migueluy@git.wordpress.org>

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[CGastrell]

Tests as described, worked with Gutenberg disabled as well. The build error is not entirely correct, the pageSlugUnderscore identifier is on a different scope, but in the same global scope? Maybe you'll need to fix that in order to make the CI checks happy