Code should be formatted using prettier

[singingwolfboy]

Using an automated formatted for code is a good idea, for several reasons:

• It ensures that the code follows a consistent style, which makes it easier to read
• It helps newcomers who are not as familiar with Javascript or Typescript to avoid errors
• It eliminates the time spent thinking about and reviewing the correct way to format code

See https://prettier.io/docs/en/why-prettier.html for more detail on these points.

In the Javascript ecosystem, Prettier has quickly become the dominant code formatter, for a variety of reasons including execution speed, GitHub stars, tool integration, and simplicity. In fact, @actions/toolkit, @actions/setup-node, @actions/typescript-action, @actions/labeler, @actions/cache, and @actions/checkout are all already using Prettier. For consistency, this project should, as well.

I've already made a pull request for this work: #16.

[bryanmacfarlane]

Fixed with #20 and #23 - thanks for pointing this out. Brings this inline with the other repos like toolkit with the same rules and approach used there.