Problem:
We want to encourage implementing core features in Lua instead of C, but
it's clumsy because:
- Core Lua code (built into `nvim` so it is available even if VIMRUNTIME
is missing/invalid) requires manually updating CMakeLists.txt, or
stuffing it into `_editor.lua`.
- Core Lua modules are not organized similar to C modules, `_editor.lua`
is getting too big.
Solution:
- Introduce `_core/` where core Lua code can live. All Lua modules added
there will automatically be included as bytecode in the `nvim` binary.
- Move these core modules into `_core/*`:
```
_defaults.lua
_editor.lua
_options.lua
_system.lua
shared.lua
```
TODO:
- Move `_extui/ => _core/ui2/`
