[FIR] Part 1. Group checkers by the MPP kind

This commit introduces MppChecker kind, which represents the new property
  of checkers
- `MppCheckerKind.Common` means that this checker should run from the same
  session to which corresponding declaration belongs
- `MppCheckerKind.Platform` means that in case of MPP compilation this
  checker should run with session of leaf platform module for sources
  of all modules

An example of a platform checker is a checker that checks class scopes
  and reports ABSTRACT_NOT_IMPLEMENTED and similar diagnostics. If some
  regular class in the common module contains expect supertypes, the
  checker should consider the actualization of those supertypes to get
  a complete type scope

^KT-58881

plugins/js-plain-objects/compiler-plugin/js-plain-objects.k2/src/org/jetbrains/kotlinx/jso/compiler/fir/checkers/FirJsPlainObjectsPluginClassChecker.kt

@@ -8,6 +8,7 @@ package org.jetbrains.kotlinx.jspo.compiler.fir.checkers
 import org.jetbrains.kotlin.descriptors.ClassKind
 import org.jetbrains.kotlin.diagnostics.DiagnosticReporter
 import org.jetbrains.kotlin.diagnostics.reportOn
+import org.jetbrains.kotlin.fir.analysis.checkers.MppCheckerKind
 import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
 import org.jetbrains.kotlin.fir.analysis.checkers.declaration.FirClassChecker
 import org.jetbrains.kotlin.fir.declarations.FirClass
@@ -26,7 +27,7 @@ import org.jetbrains.kotlin.fir.types.isAny
 import org.jetbrains.kotlin.fir.types.toRegularClassSymbol
 import org.jetbrains.kotlinx.jspo.compiler.resolve.JsPlainObjectsAnnotations
 
-object FirJsPlainObjectsPluginClassChecker : FirClassChecker() {
+object FirJsPlainObjectsPluginClassChecker : FirClassChecker(MppCheckerKind.Platform) {
     override fun check(declaration: FirClass, context: CheckerContext, reporter: DiagnosticReporter) {
         with(context) {
             val classSymbol = declaration.symbol as? FirRegularClassSymbol ?: return