Set up stricter typescript compiler settings

[spalladino]

As originally designed by @ajsantander in #434:

• [ ] sourceMaps: true // Activate source maps and, ideally, try out a debugging workflow. Might need to use the npm dependnecy "source-map-support" to be able to see references to .ts code in transpiled .js output files.
• [x] declaration: true // (not per package, but in the root tsconfig.json)
• [ ] esModuleInterop: false // Disable compatibility with Babel.
• [ ] checkJs: true // Report errors in .js files.
• [ ] alwaysStrict: true // Parse Javascript in strict mode.
• [x] allowJs: false // Stop allowing .js files.
• [ ] noUnusedLocals: true // Report errors on unused locals.
• [ ] noUnusedParameters: true // Report errors on unused parameters.
• [ ] noImplicitReturns: true // Report error when not all code paths in function return a value.
• [ ] noImplicitThis: true // this. needs to be typed always.
• [ ] noImplicitAny: true // No inference of the any type.
• [ ] strictNullChecks: true // null and undefined are strict types, to be handled by unions.
• [ ] strictFuncitonTypes: true // Disable bivariant parameter checking for function types.
• [ ] strictPropertyInitialization: true // Ensure non-undefined class properties are initialized in the constructor.
• [x] noEmitOnError: true // Do not output any Javascript if any error is found.