Support TypeScript 3.8 type keyword in import and export

[cpirich]

• TypeScript 3.8 now supports type only imports and exports (see here and the underlying PR)
• Updated the converter to take advantage of this new support
• Note: that export type * from 'module' is still not supported and neither is import A, { type B, C } from "module" - so those are still converted without the type keyword (which may continue to cause problems when compiling the converted TS files with the --isolatedModules options)