Use built types, remove custom type declarations

[chrissantamaria]

Followup to #78 - looks like the updated types aren't being recognized by package consumers, as the types field in package.json is pointing to a custom written index.d.ts in the repo root. I'm not sure if this has a purpose now (perhaps existed before this repo was natively written in TypeScript?), but I've gone ahead and removed it in favor of the built types in dist/index.d.ts.

[roderickhsiao]

Was previously added by open source before we migrated to typescript, should be good to removed.

thanks