Migrate to a typed language (e.g. TypeScript)

[jahed]

Due to the core use case of vast-parser being to transform data from one structure to another, a type system will provide a huge benefit.

The main benefit is that the structure of VastResponse can be self-documented and enforced. Right now, that structure isn't entirely obvious.

The other benefit is to auto-generate documentation (by using something like typedoc). Keeping the README up-to-date and accurate by hand isn't easy.

With a type system, it'll also make it easier to move towards a more functional approach. Eventually allowing an application to tree-shake any unused code.