feat: support v4 console access

[mattkhan]

Problem

The latest version (4.1.0 at the time of this PR) cannot be loaded in the console via init("v4.1.0")

Solution

Before: the unkpg url would be appended with /esm/index.js regardless of the version
After: if the version starts with 4, append /index.js instead

• same behavior for all versions that do not start with 4

Also defaults init() to use the current latest version.

Tested in Chrome locally

This should partially resolve #222

only partial because V3 is not supported

Alternatively, unpkg mentions:

If you omit the file path (i.e. use a “bare” URL), unpkg will serve the file specified by the unpkg field in package.json, or fall back to main.