Work on making SILE functional as a Lua library

[alerque]

No description provided.

[Omikhleia]

This old PR never came to fruition as-is and now has many conflicts, an empty description and no link to any open issue.

Due to inactivity, and as part of backlog cleaning, I would have been tempted to close it, but the topic was interesting, and the reasons why it didn't eventually occur are unclear.

I do know that recent changes (the Rust wrapper, etc.) are part of the solution towards the goal mentioned here, but the path from this PR to it are not clear.

So I'm adding the "pending closure" as a label - As a last chance for contributors to eventually say their word.
After some period of time, this inactive PR will be closed without further notice.

[alerque]

In spite of appearances I'm actively working on this. Last week in fiddling with decasify I spent extra time playing with build arrangements as a PoC for this problem (and succeeded in releasing it). In the mean time it is not practical to pursue directly until after #2072 lands.

Edit: This was even mentioned in #2072:

We're closer than ever to being able to easily be used as a library from Lua