Add CSS&JS formatting

[ckiee]

This PR adds:

• a GitHub Action to check formatting is good.
• an xtask subcommand to run prettier, pinned to the aforementioned Action's version.

[zarik5]

You should add a prettier.rc file, since different people can have a different configuration default. Also prettier.rc should be configured to keep the formatting a s close to the original code as possible (for example use 4 spaces indentation). You can pick the prettier.rc file in the new-dashboard branch.

[ckiee]

You should add a prettier.rc file, since different people can have a different configuration default.

Wrong:

Prettier intentionally doesn’t support any kind of global configuration. This is to make sure that when a project is copied to another computer, Prettier’s behavior stays the same. Otherwise, Prettier wouldn’t be able to guarantee that everybody in a team gets the same consistent results.

~prettier.io

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Also prettier.rc should be configured to keep the formatting a s close to the original code as possible (for example use 4 spaces indentation)

I chose to not do that to avoid the inevitable configuration fight and with the .git-blame-ignore-revs there is no real reason to minimize changes.

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Also, if we are moving away from the defaults I'd like to bring your attention to why tabs are important

[ckiee]

Ended up using 4 spaces after a discussion on Discord. Merging.