make the generator incremental

[akiraveliara]

makes the source generator an IIncrementalGenerator rather than an ISourceGenerator

notes:

• global usings can still theoretically break the emitted code, but the only realistic scenario i can see is global using Newtonsoft.Json causing a problem with the unqualified JsonConverter at line 130
• i have not taken the opportunity to really optimize the generator all that much. one candidate would be to replace the LINQ query at line 162 with a foreach loop.
• this doesn't address tests. for what it's worth, there should probably be tests for the generator itself; largely because debugging source generators barely works, if at all.