[monorepo js formatting] Add .prettier files to top level

Summary:
These files used to take care of everything in the web/ folder

Now that js files are in other directories, move these to top level to handle all js

Test Plan: arc lint --everything

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13589