Add a 'as_enum' on Scalar nodes to help with constraining inputs

Description

This adds helper methods on Nodes to allow getting a variable as an enum member.

This will ensure consistent error reporting for users when plugin authors want to constrain some inputs.

Changes proposed in this merge request:

• Mark as enums some types that weren't marked
• Add as_enum on ScalarNode
• Add get_enum on MappingNode as a shortcut for `mapping.get_scalar(value).as_enum(...)
• Change core to use this when custom reporting was done

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