Add basic typescript typings to javascript-montage

[FelixVicis]

Following along with the documentation https://www.typescriptlang.org/docs/handbook/declaration-files/publishing.html for adding typings to a npm package.

[FelixVicis]

doubling up on a note here to make it easier to find later:

Note of feature placement

index.d.ts should be moved from dist/ to src/ and added to the build file. The current placement is as a fix to have the expected behavior.