Add a prettier linter for the web tools

Summary:
This diff adds a web/ directory at the project root and a new linter
running prettier on all the Javascript files from this directory.

Test Plan:
Follow the installation instructions from CONTRIBUTING.md.
Add a web/dummy.js file to the source tree with some Javascript
content, then:

arc lint --trace

Check the prettier linter is run.

Edit the web/.prettierrc file to set the tab width to 4.

arc lint

Check the web/dummy.js file is updated accordingly.

Reviewers: #bitcoin_abc, deadalnix, josephroyking

Reviewed By: #bitcoin_abc, deadalnix, josephroyking

Subscribers: josephroyking, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8477