Introduce Space Style, similar to Space Script

[zefhemel]

Inspired by #786:

Perhaps we should have space-style in addition to space-script but containing CSS that is included on the page, similar to https://silverbullet.md/STYLES

For instance, as follows:

```space-style
#sb-root {
   --editor-font: "Courier" !important;
}

```

Rationale: It would be nice to be able to add custom SilverBullet CSS across multiple pages, not just one as supported right now with the customStyles setting. This would allow for https://silverbullet.md/Libraries that contain:

• Custom themes
• Custom admonitions as in Custom Admonitions #786

For the future: it would then also be nice to more easily attach custom styles to page elements. I'm open to ideas for how this could work, the most obvious candidate would be this: #702

[zefhemel]

Also related: #692

[NeilHanlon]

I am in support of this idea--specifically one of my use cases is that I often create diagrams using PlantUML and/or Mermaid syntax, and being able to display those full-screen (or side-by-side..) would be a great addition which I could implement in some way with this styling.

[zefhemel]

This is now implemented by @onespaceman