Update xo requirement to ^0.21.0
Update xo requirement to ^0.21.0

Updates the requirements on [xo](https://github.com/xojs/xo) to permit the latest version.
- [Release notes](https://github.com/xojs/xo/releases)
- [Commits](https://github.com/xojs/xo/commits/v0.21.0)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
SavageCore/new-retro-arcade-neon-attract-screen-tool,SavageCore/new-retro-arcade-neon-attract-screen-tool,SavageCore/new-retro-arcade-neon-attract-screen-tool