[tcgc] default .access to "public" instead of undefined

[iscai-msft]

Clear and concise description of the problem

Currently TCGC defaults the .access property on SdkModelType and SdkEnumTypes to .undefined unless the @access decorator is explicitly applied. This was so emitter authors could determine whether the @access was purposefully set, so if they wanted to trim these models and make them private, they could do so.

However, having an undefined default isn't the greatest, so we want to default the value "public" instead of @access is not explicitly set. If an emitter author still cares about whether the @access decorator was explicitly provided, they could call getAccessOverride from the tcgc library

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