refactor(build): include lpeg as a library

[bfredl]

This reduces the build time required luarocks packages to zero. (luarocks is now only needed for testing).

alternative: vendor lpeg as src/lpeg just like src/mpack. This avoids most extra cmake code, but vendors the lpeg c code (~5000 lines). Not sure which one will make distros recoil in horror the most.

[clason]

Do we want to make changes to lpeg for any reason? That is usually the deciding factor on whether to vendor something.

On the other hand, would vendoring allow keeping lpeg.lua out of runtime (and hence the grubby mitts of stylua)? Vendoring would also be more robust against unreachable hosts (as lpeg seems to be hosted in the private user directory on a single university server).

[dundargoc]

The cmake part looks fine apart from some nitpicks.