Added type declarations for ESM support

[staplespeter]

Type declaration generation process:

1. Install TypeScript
   >npm i typescript --save-dev

2. Create tsconfig.json

{
  "include": ["index.js", "lib/*.js"],
  "compilerOptions": {
    "allowJs": true,
    "declaration": true,
    "emitDeclarationOnly": true,
    "declarationMap": true
  }
}

3. Produce type files
   >tsc --build

4. Edit index.d.ts, adding the following as the last export
   export * from "./lib/character-sets";

Tested by using in a project, building and running on NodeJS