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.

idea/testData/quickfix/variables/changeMutability/canBeVal/const.kt

@@ -1,4 +1,5 @@
 // "Change to val" "true"
 object Test {
     <caret>const var foo = "123"
-}
+}
+/* FIR_COMPARISON */

idea/testData/quickfix/variables/changeMutability/canBeVal/const.kt.after

@@ -1,4 +1,5 @@
 // "Change to val" "true"
 object Test {
     <caret>const val foo = "123"
-}
+}
+/* FIR_COMPARISON */