build: ship "_core/*" as bytecode (built-into Nvim binary)

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/`

cmake/RunTests.cmake

@@ -69,7 +69,7 @@ endif()
 
 execute_process(
   # Note: because of "-ll" (low-level interpreter mode), some modules like
-  # _editor.lua are not loaded.
+  # _core/editor.lua are not loaded.
   COMMAND ${NVIM_PRG} -ll ${WORKING_DIR}/test/lua_runner.lua ${DEPS_INSTALL_DIR}/share/lua/5.1/ busted -v -o test.busted.outputHandlers.nvim
     --lazy --helper=${TEST_DIR}/${TEST_TYPE}/preload.lua
     --lpath=${BUILD_DIR}/?.lua