Expose LuaJIT's REPL to the interpreter CLI so that Lua chunks can be evaluated interactively

[rdw-software]

This can get complicated quickly if hand-rolled from scratch (see luvit's repl), especially with async code.

To keep things simple, should just copy/paste the LuaJIT implementation and expose it to the CLI.

Goals:

• [x] Can evaluate code in real time for basic experimentation/testing, and not just once on eval command execution
• [x] The eval command starts the REPL if it's called without any args - no need to introduce a new command for this

Future changes will be needed, but it should be sufficient to just mirror the behavior of the LJ REPL and handle async later.

[rdw-software]

Well, it seems that the quick & dirty way actually works: Copy/paste the source, modify the export signature, call dotty... done.

There's a number of eyebrow-raising things about this so I'll see if I can make the integration cleaner. Also, export to the FFI?