New formatter for frontend code

[nikitaevg]

Hi all!

I've just enabled the prettier formatter in our codebase which you can read more about on this wiki page. If you have an open PR you will probably have merge conflicts. To resolve them merge the develop branch applying your changes and then run prettier. To do it run git fetch -ap upstream && git merge upstream/develop --strategy-option ours && npx prettier . --write. Drop me a message if you struggle after this change.

Thanks,
Nikita.

UPD: If you face problems with prettier while committing code, and the problems are not related to the code format, try running ../oppia_tools/yarn-<yarn version>/bin/yarn install as per this instruction.