[ecashaddrjs] Remove version number from minified builds

Summary:
T3039

Depends on D13477

To prevent the need for manually updating the README with every release, remove the version number from minified builds. The link will now reference the latest version. Past versions can be accessed by users manually specifying the version in the URL, e.g. https://unpkg.com/ecashaddrjs@<userChangesThis>/dist/cashaddrjs.min.js

Since these builds are no longer included in the git repo, a new npm script npm run prepublish is added. This will ensure that npm run build is always run before npm run publish (and thus that the package is not inadvertently published without first building the latest version).

Now that the build files have the same name, there is no need for the npm run clean script or its dependency shx. These are removed.

Test Plan: npm run build and output files do not have version info. Newly generated file paths match the updated link in README.md.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13478