Add support for enum Values in Attributes/Properties

So the manifest should list the possible values, so a designer could use them.
also a min/max for a number could be usefull

I've the following Properties for a Property in my designer:
https://github.com/node-projects/web-component-designer/blob/master/src/elements/services/propertiesService/IProperty.ts

Start by comparing the property shape in the linked IProperty.ts file with the custom elements manifest format and its current schema. Clarify how enum values and numeric min/max values should be represented; done means the manifest can expose those values for a designer to consume.

Written by the indexing model from the issue text.