Add TypeScript type definition file for index.js and add a TS project example

[albincorreya]

Currently, we distribute the TypeScript type definition (.d.ts) files separately for each module through our releases. Though devs can use these builds for using essentia.js on a TypeScript project like this example, adding an index.d.ts type definition file for index.js can make it easy for developers to integrate into a TS project as similar as in JS. This was mentioned in #46

It would be also nice to have a TypeScript project as an example in the repository for reference.

How?

• Add index.d.ts file and specify it inside the package.json file
• Update tsconfg.json with the respective settings
• Publishing types along with the newest release of the package.
• Add an example to the repository using this typescript entry point.