v3 rewrite & migration todo list

* **CSS aggregation & compilation**
  - [x] Custom CSS import via `?css-component`
  - [x] Component list filter system

* **Component page**
  - [ ] (optional) CSS support for HTML code block
  - [x] #371

* **In-app pages**
  - [ ] Home page
  - [ ] Support page
  - User resources
    - [x] Getting started
    - [ ] itch.io's HTML quirks
    - [ ] Other resources
    - [x] Showcase