chore: migrate build system from webpack to wp-scripts

[GaryJones]

Summary

• Replaces custom 107-line webpack configuration with wp-scripts
• Aligns build tooling with other Automattic plugins (co-authors-plus, liveblog)
• Removes ~20 redundant devDependencies now provided by @wordpress/scripts
• Updates PHP enqueue calls to use auto-generated .asset.php files for dependency management
• Converts wp.* globals to ES imports in custom-status-block.js for proper dependency detection

Net change: -2,200 lines of configuration code.

Changes

File	Change
package.json	Use wp-scripts build/start, remove redundant deps
webpack.config.js	107 lines → 9 lines (extends wp-scripts default)
.babelrc.js	Deleted (wp-scripts includes babel config)
.gitignore	Add build/ folder
custom-status.php	Use .asset.php for dependencies
calendar.php	Use .asset.php for dependencies
custom-status-block.js	Convert to ES imports

Benefits

• Automatic dependency detection - No more manual dependency arrays in PHP
• Simplified maintenance - Less custom config to maintain
• Consistent tooling - Same build system as CAP and Liveblog
• Better cache busting - Content hash in .asset.php version

Test plan

• npm run build succeeds
• All 11 e2e tests pass
• Manual testing of custom status panel in block editor
• Manual testing of calendar page

🤖 Generated with Claude Code

[GaryJones]

Waiting on Automattic/eslint-config-wpvip#235 to use the correct formatting code style.