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/`

runtime/lua/vim/_init_packages.lua

@@ -95,6 +95,6 @@ end
 
 -- only on main thread: functions for interacting with editor state
 if vim.api and not vim.is_thread() then
-  require('vim._editor')
-  require('vim._system')
+  require('vim._core.editor')
+  require('vim._core.system')
 end