FIR IDE: add KtExtensionReceiverSymbol

This symbol represents the extension receiver of a callable and
currently is used as the resolution target of `this` in a callable with
extension.

analysis/analysis-api/testData/referenceResolve/thisWithLabelToFunction_caretAtLabel.kt

@@ -0,0 +1,5 @@
+class Foo {
+    fun String.foo() {
+        this@f<caret>oo
+    }
+}