Improve dependency management

[Shaptic]

This makes a few changes that should decrease bundle size:

• moves Typescript-related dependencies into the devDependencies section
• drops polyfills that aren't necessary for the package
• upgrade stellar-base to its latest beta

Design Question: There's a sentiment in the JavaScript community that "libraries shouldn't do their own polyfills," as different downstream systems can choose to handle this differently. Should we stop polyfilling entirely and just provide documentation / recommendations?

[aristidesstaffieri]

Design Question: There's a sentiment in the JavaScript community that "libraries shouldn't do their own polyfills," as different downstream systems can choose to handle this differently. Should we stop polyfilling entirely and just provide documentation / recommendations?

I see both often enough, I think it's a nice out of the box experience to not have to think about the differences but maybe in some cases(like crypto?) it would be nice to be able to swap in a different polyfill. I think in the case of http/https the dx is nice to just have those provided.