feat: Feature/custom callout icon (#727)

* Add icons as masks

To handle a simple way to add custom icons, i made it pure css. Icon are now a mask for the callout-icon div, so they always follow the --color form the current callout.

Now to add a custom icon, you simply add

```css
.callout {
  &[data-callout="custom"] {
    --color: #customcolor;
    --border: #custombordercolor;
    --bg: #custombg;
    --callout-icon: url('data:image/svg+xml; utf8, <custom formatted svg>');

  }
```

to custom.scss

* remove now unused code

* Make callouts an enum

* docs: update instructions for custom callouts

* Prettier & run format

* dynamic matching

For maintainability, make dynamic mathching. If we or Obsidian want to support more callouts, we simply add it to the enum

* callout mapping const

Getting ride of the enum entierly as it's not worth here?

* fix callout icon styling

* Add forgotten icons

* Rebase

* harmonize callout icon and fold icon

* fix docs + prettier

* Update docs/features/callouts.md

Co-authored-by: Jacky Zhao <j.zhao2k19@gmail.com>

* Update quartz/plugins/transformers/ofm.ts

Co-authored-by: Jacky Zhao <j.zhao2k19@gmail.com>

* Suggestions fix

* remove unecessary rules

* comment is always nice

* Update docs/features/callouts.md

---------

Co-authored-by: Jacky Zhao <j.zhao2k19@gmail.com>