Add support for custom npm registry url

[ghengeveld]

Fixes #512

This attempts to retrieve the current registry url using npm config get registry or the Yarn equivalent. Failing that, it falls back to the default registry url for npm.

[linear]

CH-1486 CLI custom npm registry support

#512

The company I work for disables access to the public registry, so that we use our internal registry instead - which includes a mirror of the public registry.

This causes checkForUpdates to fail with the following message:

Could not retrieve package info from registry; skipping update check

Ideally chromatic would automatically use the registry in my closest .npmrc. Alternatively this could be set with a CLI flag and/or env var.

[ghengeveld]

Released in 6.5.0.