Update default branch to `main`

[seanmakesgames]

Update to the current github standard.

[lloeki]

Related to https://github.com/rubyjs/libv8-node/issues/47

[tisba]

Looks like @SamSaffron already did the work 🥳

Since I had to look it up, here is how I changed the default branch locally:

git fetch
git checkout main
git symbolic-ref refs/remotes/origin/HEAD refs/remotes/origin/main
git branch -d master

[seanmakesgames]

yay! looks great. :D