[gitignore] Add secrets.js to monorepo .gitignore to support js apps that use keys

Summary:
T2973

Add secrets.js to the top-level .gitignore. This file is used to store information that should not go into the monorepo. For now, it is only used for Telegram bot keys.

Test Plan:

cd web/alias-server
touch secrets.js
git status

Note that this file does not appear as modified in git

cd bitcoin-abc
find . -name secrets.js

note that ./web/alias-server/secrets.js is the only output; no other secrets.js files exist in the monorepo that would be impacted by this change

Reviewers: Fabien, #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13235