#548 [Cleanup] Bump Node version, migrate to Eslint, start cleaning up lint errors

Hi, I'm a nobody in g0v and a student of Taigi. I'm using itaigi a lot and would like to give back. This PR is part of a series of changes I'd like to contribute to clean up the itaigi codebase. This PR: 1. Bumps node version from the current version (5) to the latest version (15). 2. Adds eslint as a dependency 3. Starts cleaning up the code as per eslint.