Add formatting with Prettier

[brandonbaraban]

This PR adds prettier and prettier-plugin-svelte for formatting, along with a format package.json script, and applies the formatting to the entire codebase.

With this PR, the codebase can maintain consistent styles as different people make contributions. It also makes reviewing PRs easier as changes don't include miscellaneous things like using tabs instead of spaces or vice-versa.

package-lock.json was also updated as a side effect of running npm install, because it wasn't updated since before the previous package release.

To minimize changes, the .prettierrc I added attempts to maintain most of the styles that I saw in the codebase.