feat(lua): add vim.system()

feat(lua): add vim.system()

Problem:

  Handling system commands in Lua is tedious and error-prone:
  - vim.fn.jobstart() is vimscript and comes with all limitations attached to typval.
  - vim.loop.spawn is too low level

Solution:

  Add vim.system().
  Partly inspired by Python's subprocess module
  Does not expose any libuv objects.

scripts/lua2dox.lua

@@ -340,6 +340,7 @@ function TLua2DoX_filter.filter(this, AppStamp, Filename)
 
         if vim.startswith(line, '---@cast')
           or vim.startswith(line, '---@diagnostic')
+          or vim.startswith(line, '---@overload')
           or vim.startswith(line, '---@type') then
           -- Ignore LSP directives
           outStream:writeln('// gg:"' .. line .. '"')