core.js semver version seems wrong, only considers strict major version

Went to upgrade after #31 was released and noticed that the issue continued to persist. The reason is `get-modules-list-for-target-version` is being fed `3` for the version by default, which ignores `3.1`, `3.2`, etc. I tried a few semver-y things to be more inclusive but it seems core-js doesn't support those, so one needs to manually set `3.6` or whatever the latest is, which seems less than ideal.

Start by tracing where get-modules-list-for-target-version receives its default version. Verify how core-js handles 3.x versions such as 3.1 and 3.2, then define completion as selecting the intended compatible module set without requiring users to manually specify each latest minor version.

Written by the indexing model from the issue text.