Add standard tooling configs and migrate to ESLint 9

[jeremyfelt]

Brings show-hide-section-block in line with our other plugins' tooling.

Changes

• PHPStan (level 9) — Add phpstan.neon.dist mirroring the shared happyprime/coding-standards config. composer phpstan passes; getting there required fixing the type issues it surfaced:
  • void return types on the four register() functions and register_assets()
  • a precise array-shape type for maybe_enqueue_script()'s $parsed_block
  • a stray 4th argument on a remove_filter() call (it accepts 3)
• Blueprint — Add blueprint.json for WordPress Playground (WP 7.0, activates the plugin).
• wp-env — Add .wp-env.json (WP 7.0 / PHP 8.2 / debug flags), plus @wordpress/env and env:* scripts.
• ESLint 9 — Replace legacy .eslintrc.js with flat-config eslint.config.mjs; update lint/fix scripts to eslint ..
• Browserslist — Add the standard .browserslistrc; remove the conflicting package.json browserslist field and the now-unused @wordpress/browserslist-config dep.
• .distignore — Reflect the new/removed config files.

Verification

• composer phpstan — No errors (level 9)
• composer phpcs — clean
• npm run lint — clean (ESLint 9 flat config)
• npm run build — compiles successfully

🤖 Generated with Claude Code

[github-actions]

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