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/OverridenExtensionsInObject.kt

@@ -0,0 +1,19 @@
+class T
+
+interface Foo {
+    fun T.fooExtension()
+    val T.fooProperty: Int
+}
+
+object A : Foo {
+    override fun T.fooExtension() {}
+    override val T.fooProperty get() = 10
+}
+
+fun T.usage() {
+    foo<caret>
+}
+
+// INVOCATION_COUNT: 2
+// EXIST: { lookupString: "fooExtension", itemText: "fooExtension" }
+// EXIST: { lookupString: "fooProperty", itemText: "fooProperty" }