Change files depending on firefox version

Something like `variant` but for `firefox`:

```yaml

firefox:
nightly:
repositry: https://github.com/Guerra24/Firefox-UWP-Style/archive/refs/heads/master.zip
89-90:
repository: https://github.com/Guerra24/Firefox-UWP-Style/archive/refs/heads/firefox-89.zip
88:
repository: https://github.com/Guerra24/Firefox-UWP-Style/archive/refs/heads/firefox-88.zip
76-87:
repository: https://github.com/Guerra24/Firefox-UWP-Style/archive/refs/heads/firefox-76.zip
```

Now `firefox: ` (see #13) will need to be transformed before unmarshaling to:

```yaml
firefox:
: {}
```

To get the compatible firefox versions, just `firefox`'s keys as OR patterns.

By the way, we got support OR patterns with `firefox: [53-65, 89+]` (that gets expanded to `firefox: {53-65: {}, 89+: {}}` before unmarshaling) (see #16)