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/repeatedModifiers.fir.kt

@@ -39,3 +39,5 @@ fun test() {
     }
 }
 
+<!WRONG_MODIFIER_TARGET!>external<!> <!REPEATED_MODIFIER!>external<!> val i = 0
+<!WRONG_MODIFIER_TARGET!>const<!> <!REPEATED_MODIFIER!>const<!> var x = 0