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_.

.github/workflows/ci.yml

@@ -205,6 +205,8 @@ jobs:
     env:
       CC: ${{ matrix.cc }}
       CI_OS_NAME: ${{ matrix.os }}
+      # TEST_FILE: test/functional/core/startup_spec.lua
+      # TEST_FILTER: foo
     steps:
       - uses: actions/checkout@v3
 
@@ -281,6 +283,8 @@ jobs:
       DEPS_BUILD_DIR: ${{ github.workspace }}/nvim-deps
       CACHE_NVIM_DEPS_DIR: ${{ github.workspace }}/nvim-deps
       DEPS_PREFIX: ${{ github.workspace }}/nvim-deps/usr
+      # TEST_FILE: test/functional/core/startup_spec.lua
+      # TEST_FILTER: foo
     name: windows (MSVC_64)
     steps:
       - uses: actions/checkout@v3