Incremental processing: support getSealedSubclasses from multiple files

There isn't a simple way to associate symbols with their supertypes
without resolution, because of typealias. Therefore, all sealed
classes / interfaces on which getSealedSubclasses is called are
invalidated.

Synthesize an unique name for processor classpath

Label properties explicitly for Gradle

Delete unnecessary / confusing gradle.properties in test

make KSP return AnalysisResult.internalError for KSP internal errors …

…instead of throwing exceptions

Use JDK8 by default in test and releases

Gradle sets required org.gradle.jvm.version by JAVA_HOME. Therefore,
building KSP with JDK11 produces binaries that depend on Java 11 at
minimum.

update reference.md for alpha

Add library support table and other doc updates

1. Library support table
2. Update README to include latest speedup stats (up to 2x)
3. Fix broken playground link in quickstart