Reorganize squiggle-language lib into different export paths

[OAGr]

Right now, squiggle-language directly exports a long list of different types/functions/errors. It could be neater to divide this into a few subdirectories.

I imagine we might want:

• /errors
• /values
• /utility

and maybe a few others

[berekuk]

What's the expected benefit of this?

I know barrel files can be bad for performance, but my impression is that squiggle-lang is small enough for this to matter, and I don't expect that we can tree-shake squiggle-lang because SqProject depends on everything else.