Enable the renovate config in this repo to be used outside of the adobe org

**Is your feature request related to a problem? Please describe.**

I would like to be able to reuse the renovate configuration from `default.json` in a repository that is **not** part of the `adobe` org. However, this does not work because the `npmToken` is contained in this file and renovatebot (correctly) refuses to decrypt this for git repositories outside of this org.

> Note - overriding this configuration does **not** work; renovatebot always tries to decrypt the values from extended configurations.

**Describe the solution you'd like**

1. Extract everything *except* the encrypted token into a separate file (let's say `core.json`)
2. Change `default.json` to be
```
{
extends: [
'local>adobe/helix-shared:core'
],
encrypted: {
npmToken: '...'
}
}
```

Then a repository outside of this org can have

```
{
extends: [
'github>adobe/helix-shared:core'
],
...more config...
}
```

And not run into encryption issues.

**Describe alternatives you've considered**

1. Copy/paste 😢
2. Move token to `renovate.json5` -- this, however, would break backwards compatibility for repositories in the adobe org.

**Additional context**

Happy to submit a PR for this but wanted to first understand if it is desirable to have this configuration be reusable outside of this org.

Start by locating default.json and reviewing how its current extended configuration is consumed. Separate the reusable configuration into core.json as proposed, then verify that external repositories can extend core without decrypting npmToken while Adobe repositories remain compatible with default.json.

Written by the indexing model from the issue text.