Add support for customMetas and customDefines fields

[kLabz]

• Implement a check similar to https://github.com/HaxeFoundation/haxelib/blob/development/src/haxelib/Data.hx#L252-L264 for documentation.defines / documentation.metadata fields (if set)
• Update json schema
• Open PR for updated json schema on https://github.com/SchemaStore/schemastore

Those fields will be used in HaxeFoundation/haxe#10858

[tobil4sk]

Really neat stuff! :D This will solve #262.

We might also want to add some validation, so that when a library is submitted we make sure the files actually exist and that they contain valid data, since we do that with haxelib.json.

[Simn]

This looks good to me.

@tobil4sk feel free to merge if there's nothing else to do!