Rename primary branch to main

[mhdawson]

We going through all of the node.js repositories to rename the primary branch to main. Please see https://github.com/nodejs/node/issues/33864 for more context.

I don't think there should be any issues for the contributors to this repo. I'd like to do the remain this Friday May 15th unless there are any concerns.

Please let me know in this issue before May 15th if there are any concerns, otherwise I'll go ahead and do the change.