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

[tyru]

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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

https://github.com/vim-jp/vim-vimlparser/issues/118#issuecomment-513278223

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).