Check missing/added Ex commands in neovim *by script*

[tyru]

Currently

• s:VimLParser.neovim_additional_commands
• s:VimLParser.neovim_removed_commands

is managed manually.
neovim supports nvim_get_commands() listing available commands.

So it can automate by adding script diffing command list.

Ref

#118 (comment)

However, nvim_get_commands exists already (although only for non-builtin). Might be useful in general, but only for "dynamic" linting/parsing (i.e. not for vint, vimlparser.py etc, which need the list already).