Allow scripts and targets in schema to be permissive

[devnote-dev]

Geode makes use of custom properties in these fields which show as invalid in the editor. In future it would probably be a good idea to also make other parts of the schema permissive.

[ysbaddaden]

Well, they're custom extensions that don't conform to the SPEC. The schema error is right to outline these as such.

Maybe the SPEC could evolve to allow for some extensions (in a future compatible way).

[devnote-dev]

Geode's primary use case for targets is allowing a flags: property to be set which is passed with the command when building a target, and for scripts is allowing custom scripts (platform-dependent or not) to be ran, including an extension of postinstall for platform support.

These are of course subject to change during development so I'm not sure how the SPEC could be changed to allow extensions, besides just labeling them unsupported by Shards.

[straight-shoota]

I don't think we can allow arbitrary additional properties in the schema. They could conflict with future properties added to the specification, undermining its purpose.
Shards itself does not prevent you from adding properties and it'll happily ignore anything it doesn't recognize. This is probably reasonable behaviour in the interest of maximum compatibility. However we should not bless any additions outside the specification as being conform with the the schema.