Rename `master` branch to `main`

[hlxid]

Description

More and more projects of the open-source community are changing the default git branch name from master to main.
For more information about this refer to github/renaming.

On most other projects I work on the branch is already named main. At this point everytime I do something on nodecg-io I need to used to master again because I type main automatically. Therefore I want to move to main here aswell.

TODO list:

• Ensure that nodecg-io-cli can clone repositories where the default branch is not master, otherwise it would not be able to create dev installs after the renaming
• Update GitHub Action workflows
  • Update Action workflows and docs script for master to main renaming #299
  • Update docs and Action workflows for master to main renaming nodecg-io-docs#82
  • Update GitHub Action workflows for master to main renaming nodecg-io-cli#26
• Rename branches
  • codeoverflow-org/nodecg-io
  • codeoverflow-org/nodecg-io-docs
  • codeoverflow-org/nodecg-io-cli
• Post a message on discord with instructions on how to update local git branches
  • GitHub automatically does this when someone visits a repository after renaming, but I suspect some will just perform pull and not go to the GitHub everytime they update their install