Add TypeScript typing files for htm

[jviide]

This pull request adds basic TypeScript type declarations for htm.

The typings are rather minimal. Only htm's default export is typed, and it's just an object with one method: bind(h). Here h is the pragma function and its argument types are quite lax for convenience's sake. bind's result is a function whose return type is based on h's return type:

// const html: (strings: TemplateStringsArray, ...values: any[]) => number | number[]
const html = htm.bind((type, props, ...children) => {
   return children.length;
});

The type declarations are located in src/index.d.ts. That file is then copied to dist/index.d.ts and mini/index.d.ts as a part of the build process, after Microbundle has done its magic.