Implements a `require` function that supports built-in modules like so:
```lua
local log = require('devilutionx.log')
```
It falls back to reading from assets, so this loads `lua/user.lua`:
```lua
local user = require('lua.user')
```
The bytecode for the asset scripts is cached, in case we want to later
support multiple isolated environments.
There may be a simpler or better way to do this.
It's good enough for now until someone more knowledgeable
about Lua comes along.
Gleb Mazovetskiy