[create-block] Updates variant handling for missing directory files

[bacoords]

What?

Fixes external template variants not loading files in @wordpress/create-block

When using an external npm template (like @woocommerce/create-woo-extension) with the --variant flag, variant-specific template files were not being scaffolded, resulting in empty directories. This only occurred when pulling the templates from NPM, if you try to pull a template from your local machine (i.e. path/to/woocommerce/create-woo-extension), it doesn't have the same issue because the files are persistent on your machine.

Note: this PR was drafted with the help of Claude Code.

Related: #70855

Why?

External templates are downloaded to a temporary directory, loaded via require(), then the temp directory is immediately deleted. The bug occurred because:

1. The main template files were processed while the temp directory existed
2. Variant-specific template paths were stored but not processed
3. The temp directory was cleaned up via rimraf()
4. Later, when scaffolding tried to read variant files from those paths, the directories no longer existed

This made it impossible to use variants with external templates, severely limiting the extensibility of @wordpress/create-block.

How?

The fix ensures all variant template files are loaded before the temporary directory is deleted:

1. templates.js - Modified configToTemplate() to eagerly process all variant template paths and return a variantTemplates object containing pre-loaded template content
2. scaffold.js - Updated to use pre-processed variant templates when available, with fallback to legacy path-based loading for backward compatibility
3. index.js - Fixed variant parameter passing in quick mode to ensure the variant name reaches the scaffold function

Testing Instructions

Test 1: External template with custom variant

1. Run: npx path/to/local/gutenberg/packages/create-block -t @woocommerce/create-woo-extension --variant=sql-modification test-variant
2. Verify the plugin directory is created with all files (should include composer.json, includes/Admin/Setup.php, tests/Test.php, etc.)
3. Check src/ directory contains variant-specific JS files

Before: Directory would be created but most files missing, variant directories empty
After: All 14 plugin template files and 2 block template files created correctly

Test 2: External template with default variant

1. Run: npx path/to/local/gutenberg/packages/create-block -t @woocommerce/create-woo-extension test-default
2. Verify all default template files are created
3. Ensure no regression from the fix

Test 3: Built-in template variants (regression test)

1. Run: npx path/to/local/gutenberg/packages/create-block -t es5 --variant=dynamic test-builtin
2. Verify the dynamic variant files are created, including render.php
3. Confirm built-in templates still work as expected

Testing Instructions for Keyboard

Not applicable - this is a CLI tool with no UI changes.

Screenshots or screencast

Before:

$ npx @wordpress/create-block -t @woocommerce/create-woo-extension --variant=sql-modification test
# Variant directories show as empty:
/tmp/wp-create-block-xxx/variants/sql-modification
[]
/tmp/wp-create-block-xxx/variants/sql-modification/src
[]

After:

$ npx @wordpress/create-block -t @woocommerce/create-woo-extension --variant=sql-modification test
# All variant files successfully scaffolded:
test/
├── composer.json
├── includes/Admin/Setup.php
├── languages/woo-plugin-setup.pot
├── src/index.js
├── src/index.scss
├── tests/Test.php
└── ... (14 total files)

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

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