Migrate to TypeScript

[dipasqualew]

The repository is currently using Flow for static type checking. It should use TypeScript instead.

Requirements:

• Library is fully written in TS (no JS files apart from node modules).
• Build script compiles TS into JS.
• Compiled JS is side-effects free (required by tree-shaking).
• Compiled JS exposes Individual modules (required by tree-shaking).
• Compiled JS includes types.
• Compiled JS includes source maps.

As this is a major overhaul of the library, it should be marked as a breaking change and version should bump to 2.0.0