feat(lua)!: execute Lua with "nvim -l"

Problem:
Nvim has Lua but the "nvim" CLI can't easily be used to execute Lua
scripts, especially scripts that take arguments or produce output.

Solution:
- support "nvim -l [args...]" for running scripts. closes #15749
- exit without +q
- remove lua2dox_filter
- remove Doxyfile. This wasn't used anyway, because the doxygen config
  is inlined in gen_vimdoc.py (`Doxyfile` variable).
- use "nvim -l" in docs-gen CI job

Examples:

    $ nvim -l scripts/lua2dox.lua --help
    Lua2DoX (0.2 20130128)
    ...

    $ echo "print(vim.inspect(_G.arg))" | nvim -l - --arg1 --arg2
    $ echo 'print(vim.inspect(vim.api.nvim_buf_get_text(1,0,0,-1,-1,{})))' | nvim +"put ='text'" -l -

TODO?
  -e executes Lua code
  -l loads a module
  -i enters REPL _after running the other arguments_.

1 files changed, 34 insertions, 0 deletions

diff --git a/test/functional/fixtures/startup.lua b/test/functional/fixtures/startup.lua
new file mode 100644
index 0000000000..37e738c75e
--- /dev/null
+++ b/test/functional/fixtures/startup.lua
@@ -0,0 +1,34 @@
+-- Test "nvim -l foo.lua …"
+
+local function printbufs()
+  local bufs = ''
+  for _, v in ipairs(vim.api.nvim_list_bufs()) do
+    local b = vim.fn.bufname(v)
+    if b:len() > 0 then
+      bufs = ('%s %s'):format(bufs, b)
+    end
+  end
+  print(('bufs:%s'):format(bufs))
+end
+
+local function parseargs(args)
+  local exitcode = nil
+  for i = 1, #args do
+    if args[i] == '--exitcode' then
+      exitcode = tonumber(args[i + 1])
+    end
+  end
+  return exitcode
+end
+
+local function main()
+  printbufs()
+  print('args:', vim.inspect(_G.arg))
+
+  local exitcode = parseargs(_G.arg)
+  if type(exitcode) == 'number' then
+    os.exit(exitcode)
+  end
+end
+
+main()