FIR IDE: add completion support to classes and to type aliases

idea/idea-completion/testData/basic/common/inInitBlock/localVariable.kt

@@ -0,0 +1,7 @@
+class X {
+    init {
+        val y = 1
+        <caret>
+    }
+}
+// EXIST: y

idea/idea-completion/testData/basic/common/inInitBlock/property.kt

@@ -0,0 +1,9 @@
+class X {
+    val x: Int = 10
+
+    init {
+        <caret>
+    }
+}
+// EXIST: x
+// FIR_COMPARISON