KT-7715 Highlight var's that can be replaced by val's

#KT-7715 Fixed

idea/testData/quickfix/variables/changeMutability/canBeVal/.inspection

@@ -0,0 +1 @@
+org.jetbrains.kotlin.idea.inspections.CanBeValInspection

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

@@ -0,0 +1,9 @@
+// "Make variable immutable" "true"
+fun foo(p: Int) {
+    <caret>var (v1, v2) = getPair()!!
+    v1
+}
+
+fun getPair(): Pair<Int, String>? = null
+
+data class Pair<T1, T2>(val a: T1, val b: T2)

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

@@ -0,0 +1,9 @@
+// "Make variable immutable" "true"
+fun foo(p: Int) {
+    <caret>val (v1, v2) = getPair()!!
+    v1
+}
+
+fun getPair(): Pair<Int, String>? = null
+
+data class Pair<T1, T2>(val a: T1, val b: T2)

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

@@ -0,0 +1,5 @@
+// "Make variable immutable" "true"
+fun foo(p: Int) {
+    <caret>var v: Int
+    if (p > 0) v = 1 else v = 2
+}

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

@@ -0,0 +1,5 @@
+// "Make variable immutable" "true"
+fun foo(p: Int) {
+    <caret>val v: Int
+    if (p > 0) v = 1 else v = 2
+}