[monorepo dir org] Move ecashaddrjs to modules

Summary:
T3067

Incremental piece of desired end state of monorepo organization of web/ folder.

ecashaddrjs will be a supported npm module going forward. Initialize a modules/ folder and move ecashaddrjs into it.

Update .arclint to support its new home.

Test Plan: cd modules/ecashaddrjs, npm test pass

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13559