Prettier settings are not applied

The [.prettierrc](https://github.com/isomorphic-git/lightning-fs/blob/main/.prettierrc) file specifies `semi: true` which [means that](https://prettier.io/docs/en/options.html#semicolons) a semi colon should finish each line, yet many files (for example [HttpBackend.js](https://github.com/isomorphic-git/lightning-fs/blob/main/src/HttpBackend.js)) do not have trailing semicolons, or only have one or two.

Seems like prettier is not actually being used. Would you like a PR to fix this across all the files?