[jnigen] Kotlin interfaces with suspend methods

[liamappelbe]

• In dart_generator.dart, I factored out some of the logic for dealing with async methods into a new Method extension
• Used that extension to add suspend support to _ConcreteImplClosureDef, _AbstractImplFactoryArg, _ConcreteImplClosureCtorArg, and _ConcreteImplMethod
• _ConcreteImplMethod is where the interesting stuff happens. It constructs a KotlinContinuation, invokes the user's method, then passes the future to KotlinContinuation.resumeWithFuture.
• KotlinContinuation is a new internal class with some helpers for resuming continuations.
• Continuations can be resumed with an arbitrary value, an exception wrapped in a kotlin/Result$Failure, or a value wrapped in a kotlin/Result. The existing suspend function invocation logic didn't handle that last case, so I also changed _MethodGenerator to add support for this case. I don't know how common this is in practice though.

Fixes #558

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Breaking changes ⚠️

Package	Change	Current Version	New Version	Needed Version	Looking good?
jni	Breaking	0.15.2	0.15.3-wip	0.16.0 Got "0.15.3-wip" expected >= "0.16.0" (breaking changes)	⚠️

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[coveralls]

coverage: 87.037% (-5.1%) from 92.127%
when pulling 79cc033 on jnigen_susitf
into 4341e37 on main.