KT-15286: Provide object member extensions in the completion

- Add extensions only when completion of static members is enabled (double ctrl + space and nonempty prefix)
- Add full import for callables with receiver in `LookupElement.decorateAsStaticMember`

idea/idea-completion/testData/basic/common/extensionMethodInObject/ObjectExplicitReceiver.kt

@@ -0,0 +1,14 @@
+class T
+
+object A {
+    fun T.fooExtension() {}
+    val T.fooProperty get() = 10
+}
+
+fun usage(t: T) {
+    t.foo<caret>
+}
+
+// INVOCATION_COUNT: 2
+// EXIST: { lookupString: "fooExtension", itemText: "fooExtension" }
+// EXIST: { lookupString: "fooProperty", itemText: "fooProperty" }