feat(lua): low-level interpreter mode (nvim -ll)

runtime/doc/starting.txt

@@ -239,6 +239,14 @@ argument.
 		Disables |shada| unless |-i| was given.
 		Disables swapfile (like |-n|).
 
+							*-ll*
+-ll {script} [args]
+		Execute a lua script, similarly to |-l|, but the editor is not
+		initialized. This gives a lua enviroment similar to a worker
+		thread. See |lua-loop-threading|.
+
+		Unlike `-l` no prior arguments are allowed.
+
 							*-b*
 -b		Binary mode.  File I/O will only recognize <NL> to separate
 		lines.  The 'expandtab' option will be reset.  The 'textwidth'

runtime/lua/vim/_init_packages.lua

@@ -42,8 +42,11 @@ function vim._load_package(name)
   return nil
 end
 
--- Insert vim._load_package after the preloader at position 2
-table.insert(package.loaders, 2, vim._load_package)
+-- TODO(bfredl): dedicated state for this?
+if vim.api then
+  -- Insert vim._load_package after the preloader at position 2
+  table.insert(package.loaders, 2, vim._load_package)
+end
 
 -- builtin functions which always should be available
 require('vim.shared')
@@ -78,6 +81,6 @@ function vim.empty_dict()
 end
 
 -- only on main thread: functions for interacting with editor state
-if not vim.is_thread() then
+if vim.api and not vim.is_thread() then
   require('vim._editor')
 end