Access to SILE as Lua library doesn't work as advertized

[alerque]

At some point we broke the access to using SILE as a library without quite a few undocumented shims. Too many things that are not CLI specific have been moved into sile.in (path handling, version setting) than need to be abstracted into something that core/sile.lua handles.

[alerque]

See path handling and version stuff that shouldn't be needed in https://sile-typesetter.org/examples/byhand.lua.

[alerque]

c.f. sile-typesetter/sile-typesetter.github.io#40