External markdown enhancements

[StevenClontz]

• [x] - On /dev/preview, don't show the pi-Base markdown (duplicative of the textbook entry)
• [x] - Show the name of the space/property for {S123} and {P123} citations.
• [x] - Show the theorem for {T123} citations
• [x] - Support {S123|P456} traits.

[StevenClontz]

For the first, a hint: use this in packages/viewer/src/components/Shared/Source.svelte to add optional internal and external attributes to the Source component

  export let internal = false
  export let external = false

[StevenClontz]

More hints:

import context from '@/context'
const { spaces } = context()

const name = $spaces.find(`S${id}`)?.name

[StevenClontz]

And {S000123|P000123} should link to /spaces/S000123/properties/P000123

[StevenClontz]

Closed by #193