Support adding incubator modules via the Gradle plugin

[jeffalder]

Fixes #177

I don't love this approach but there are some constraints:

1. Because the plugin is independent of the JavaFX version, I expect that JavaFX will potentially incubate other modules that become top-level modules. So the enum namespace had to incorporate potentially both incubator and non-incubator modules. Hence the _INCUBATOR name. However, this exceeds the bounds of what's known, so perhaps this is not specifically necessary.
2. That said, keying off a name (e.g., asserting that all incubator modules have _INCUBATOR attached) is not ideal. That's why there's an explicit boolean, default false, that also indicates that it's an incubator.

Other approaches considered:

• Not doing this at all, but as explained in Cannot use incubator modules #177, using incubator modules becomes challenging.
• As in point 2 above, dropping the boolean and keying off the name
• Simply hardcoding the incubator module and artifact names instead of calculating them (in fact, we should consider just doing that for all these items).
• Switching the enums to lower-case in order to avoid a locale-sensitive operation like changing the case of I.
• Dropping the enum entirely and building a Map<String, JavaFXModule> where the key is the module name and the record contains all the necessary data fields. I didn't think that was level of rewrite was necessarily beneficial, and it would raise the minimum Java version of the plugin from 11 to 16.