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/testData/diagnostics/tests/modifiers/const/applicability.fir.kt

@@ -4,7 +4,7 @@ import kotlin.reflect.KProperty
 
 const val topLevel: Int = 0
 const val topLevelInferred = 1
-const var topLeveLVar: Int = 2
+<!WRONG_MODIFIER_TARGET!>const<!> var topLeveLVar: Int = 2
 
 private val privateTopLevel = 3
 

compiler/testData/diagnostics/tests/modifiers/const/types.fir.kt

@@ -12,6 +12,6 @@ const val arrayConst: Array<String> = arrayOf("1")
 const val intArrayConst: IntArray = intArrayOf()
 
 const val unresolvedConst1 = <!UNRESOLVED_REFERENCE!>Unresolved<!>
-const var unresolvedConst2 = <!UNRESOLVED_REFERENCE!>Unresolved<!>
+<!WRONG_MODIFIER_TARGET!>const<!> var unresolvedConst2 = <!UNRESOLVED_REFERENCE!>Unresolved<!>
 const val unresolvedConst3 = <!PROPERTY_INITIALIZER_NO_BACKING_FIELD, UNRESOLVED_REFERENCE!>Unresolved<!>
 get() = 10