npm 7 compatibility

[benmccann]

Fixes https://github.com/pubkey/broadcast-channel/issues/604

I can't build this library locally because I have npm 7 installed and npm 7 checks for conflicting package versions.

npm ERR! code ERESOLVE
npm ERR! ERESOLVE unable to resolve dependency tree
npm ERR!
npm ERR! While resolving: [email protected]
npm ERR! Found: [email protected]
npm ERR! node_modules/rollup
npm ERR! dev rollup@"2.53.2" from the root project
npm ERR!
npm ERR! Could not resolve dependency:
npm ERR! peer rollup@">=0.66.0 <2" from [email protected]
npm ERR! node_modules/rollup-plugin-uglify
npm ERR! dev rollup-plugin-uglify@"6.0.4" from the root project
npm ERR!

[pubkey]

Thanks, merged.