Code golf with modern syntax & features

[jamiebuilds]

I don't know what you intend the browser support to be for this package, but Google Analytics own browser support is limited to very modern browsers, and using some modern features can remove a decent number of bytes from this package.

[jamiebuilds]

The ??= syntax isn't supported by the version of terser used in bundt so I'll make a PR over there to bump it

[jamiebuilds]

PR over here lukeed/bundt#10

[jamiebuilds]

Tried it with bundt@next:

  src/index.js             Filesize  (gzip)
    dist/ganalytics.js       348 B   265 B 
    dist/ganalytics.mjs      351 B   267 B