Reduce the bundle size

Currently requiring `stackdriver-errors-js` results in quite a lot of code being pulled in -- it's one of our largest dependencies.

The above is generated using https://github.com/webpack-contrib/webpack-bundle-analyzer.

Stat Size: 136.64KB (before minification)
Parsed Size: 39.06 KB (after minification)
**Gzipped: 11.65KB (after minification + gzip)**

Although it's not strictly because of code in `stackdriver-errors-js`, I figured this is a good place to start to get an understanding of what really is required to report errors to stackdriver.

In particular I'd love to find out how important `stacktrace-js` is to this libraries functionality and whether there are opportunities to cut it or use an alternative dependency -- I wonder if it's supporting very old browsers / edge cases which are no longer relevant to everybody 🤔

I'm tempted to just try to write a simple stackdriver API wrapper without `stacktrace-js` but of course there's every chance I'll end up going through the exact same process this library went through and realise it's not possible without those large dependencies so any advice is welcome!