Add prettier config

[toger5]

This adds a prettier config.
Before this format on save changed a lot of lines.

[toger5]

I did two commits one with the changes and one with just yarn prettier:format

[sonarqubecloud]

Quality Gate failed

Failed conditions
64.2% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud

[AndrewFerr]

For some reason I'm able to get a slightly smaller diff by running Prettier on the original source:

git checkout master -- src/ test/ && yarn prettier:format && git add src/ test/

You can use it if you like.

[AndrewFerr]

I realize Element Call has a VSCode settings file too, but I'm not a fan of putting IDE config files in version control, as they can clash with personal development preferences.

[AndrewFerr]

(To be clear, the request is to remove this file 🙂)

[AndrewFerr]

It should also be made clear that the code coverage test fails simply because this PR touches most source/test files, and the code coverage %age of the whole codebase is lower than the minimum per-PR coverage requirement...meaning 1) this is a false-positive and 2) code coverage should eventually be improved in general so this won't be an issue anymore!