feat: default value attribute

[vicdotexe]

Description

Implemented DefaultAttribute to enable initial values on model properties.

Fixes #297

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• Created tests which fail without the change (if possible)
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[ascott18]

As I've thought about this more, I'm not sure this is the right solution to the problem at hand. I think the real problem here is the implicit required rule that I added for numbers whose range excludes zero after this discussion:

ce71b21#diff-3c5b2db008ce784d0530445b78d13bf07bd3e855765300a23205359a1d461ceaR61-R80

Populating default values in ModelConversionVisitor may cause unexpected issues that I don't think we want. There fundamental assumptions in coalesce-vue that convertToModel({}, meta) and likewise for mapToModel produce an object whose fields are all null.

If we do want to support DefaultValueAttribute, I think it should probably be a feature that only affects admin pages in create scenarios.

[ascott18]

Pushed some changes and will merge momentarily. The major change is to only populate default values in the viewmodel constructor, as this allows for prop dirty flags to behave as expected as well as preventing default values from interfering with other usages of models, like API response objects.