Add TypeScript type definitions

[theraccoonbear]

This is my first time writing type defs for an existing JS module. In my testing this provides auto-completion in VSCode and makes the TS compiler happy when importing the module.

Closes #25

[infusion]

Looks pretty complete now. I don't know if you want to support the constructor (numerator: number, denominator: number); syntax for all operator methods like add, sub, ... as well. The second thing is, you had the module name Fraction before. I like this better than with the .js suffix in the ts world. Does this cause any problems?

[theraccoonbear]

Good call. I've updated the methods to support the same arguments as the constructor. I also renamed the module to Fraction. Again, I am new to this, but I don't think the module name in this type def is strictly required.

[infusion]

That looks pretty! Do you still have something in the making? Otherwise I would merge.

[theraccoonbear]

@infusion I think this is complete at this point. Please merge whenever you have a chance. Thanks!

[infusion]

Thank you!