Mark Generator-Attributes with the `[Conditional]` Attribute

[Flash0ver]

The Generated*Attributes in the FlashOWare.Generators are only markers to trigger the incremental source generators.
They should only be included in the final compilation if explicitly requested.

See ConditionalAttribute Class.

Question: What should the ConditionString be?

• e.g. [Conditional("FLASHOWARE_GENERATORS")]
• e.g. [Conditional("INCLUDE_FLASHOWARE_GENERATOR_ATTRIBUTES")]
• e.g. [Conditional("INCLUDE_FLASHOWARE_GENERATORS_ATTRIBUTES")]