Change default branch name

[nex3]

We've changed the rest of the core Sass team's repos to use main as their default branch, but changing it for this repository is particularly tricky because Homebrew uses Git as part of its infrastructure and will be broken if we just get rid of master entirely (see Homebrew/brew#7746). I'm not super comfortable with the solution proposed in Homebrew/brew#7746 (comment) of leaving a fake formula in the old branch, but we can follow the general principle by:

• Making main the new default branch.
• Adding a GitHub action to update master every time main gets a new commit.
• (Optionally) adding a note to master indicating that users should move off of it and onto main instead.