[Analysis API] Implement separate caching for unstable dangling files

PSI events do not arrive for dangling files with no backing
'VirtualFile', so its partial invalidation becomes non-trivial.
In the change, a short-living cache is implemented for these 'unstable'
dangling files.

analysis/project-structure/src/org/jetbrains/kotlin/analysis/project/structure/KtModule.kt

@@ -240,6 +240,13 @@ public interface KtDanglingFileModule : KtModule {
         get() = "Temporary file"
 }
 
+/**
+ * True if the dangling file module supports partial invalidation on PSI modifications.
+ * Sessions for such modules can be cached for longer time.
+ */
+public val KtDanglingFileModule.isStable: Boolean
+    get() = file.isPhysical && file.viewProvider.isEventSystemEnabled
+
 /**
  * A set of sources which live outside the project content root. E.g, testdata files or source files of some other project.
  */