Add neovim v0.3.1

[ericbn]

as now it's being distributed as a macOS binary, and we don't need to exclusively depend on the many-required-dependencies homebrew-core formula anymore.

After making all changes to the cask:

• brew cask audit --download {{cask_file}} is error-free.
• brew cask style --fix {{cask_file}} reports no offenses.
• The commit message includes the cask’s name and version.
• The submission is for a stable version or documented exception.

Additionally, if adding a new cask:

• Named the cask according to the token reference.
• brew cask install {{cask_file}} worked successfully.
• brew cask uninstall {{cask_file}} worked successfully.
• Checked there are no open pull requests for the same cask.
• Checked the cask was not already refused.
• Checked the cask is submitted to the correct repo.

[ericbn]

The build failed with:

audit for neovim: warning
 - possible duplicate, cask token conflicts with Homebrew core formula: https://github.com/Homebrew/homebrew-core/blob/master/Formula/neovim.rb
style
2.84s==> brew cask style neovim ✘
style check failed

I'm assuming it's not an issue to have a duplicate name for a duplicate app with homebrew-core, as it's the intention here to have the binary distribution of the same app.

Regarding the "style check failed", that's not what I get locally:

%  brew cask style --fix Casks/neovim.rb

1 file inspected, no offenses detected

[ShivaHuang]

There are issues with cask style. Please reference to #52775

[vitorgalvao]

CLI-only open-source tool, so we can’t accept it. This policy is documented in faq/rejected_casks:

The app is both open-source and CLI-only (i.e. it only uses the binary artifact). In that case, and in the spirit of deduplication, submit it first to Homebrew/core as a formula that builds from source. If it is rejected, you may then try again as a cask (link us to the issue so we can see the discussion and reasoning for rejection).