Publish type declarations

speedscope is written in TypeScript, but when you `npm install speedscope`, you only get the generated JavaScript, not any of the type declarations (`.d.ts` files).

I have some TypeScript code in my application to convert profile data from pyinstrument format to speedscope's format. So being able to import the types in `file-format-spec.ts` would be quite helpful for getting this right.

This could be achieved either by putting `file-format-spec.ts` (which has no dependencies) in the npm release, or by adding `--declaration` to the `tsc` command to force it to output `.d.ts` files.

Related to #16