[ecashaddrjs] linting with eslint

Summary:
T3044

To bring ecashaddrjs into arc lint, first lint the repo with eslint

Test Plan:
Confirm changes are linting errors
eslint . from web/ecashaddrjs/ and no output

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13546