FIR checker/IDE: Add checker and quickfix for WRONG_MODIFIER_TARGET.

There are only simple checks for `external` and `const` for now. The
rest of the checks (see ModifiersChecker in FE1.0) will be added later.

compiler/fir/entrypoint/src/org/jetbrains/kotlin/fir/checkers/CommonDeclarationCheckers.kt

@@ -18,6 +18,8 @@ object CommonDeclarationCheckers : DeclarationCheckers() {
         FirModifierChecker,
         FirConflictsChecker,
         FirConflictingProjectionChecker,
+        FirExternalDeclarationChecker,
+        FirConstDeclarationChecker,
     )
 
     override val memberDeclarationCheckers: Set<FirMemberDeclarationChecker> = setOf(