prettier config

prettier config

[Prettier][1] is an up-to date code formatter for JavaScript ecosystem.

For settings we rely on [EditorConfig][2] for things like tab style and
size (with added bonus that the code editor with an EditorConfig plugin
does some automated code formatting on file save for you). Unfortunately,
Prettier's Glob handling isn't great:
 1. `*.{ts,js,json}` has no effect
 2. Similarly, in a list of globs `*.ts,*.js,*.json` only the first glob
has an effect, the rest are ignored.
That's why the file looks the way it does.

The only other setting we change is line width. [Lukas][3] suggested we
use 100 instead of 80, because that's what Rustfmt is using.

[1]: https://prettier.io
[2]: https://editorconfig.org
[3]: https://github.com/Veykril