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/thisExtensionReceiver.txt

@@ -0,0 +1,2 @@
+Resolved to:
+0: extension receiver of fun kotlin.String.foo()

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

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

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

@@ -0,0 +1,2 @@
+Resolved to:
+0: (in Foo) fun kotlin.String.foo()

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

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

analysis/analysis-api/testData/referenceResolve/thisWithLabelToFunction_caretAtThis.txt

@@ -0,0 +1,2 @@
+Resolved to:
+0: extension receiver of fun kotlin.String.foo()

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

@@ -0,0 +1,6 @@
+class Foo {
+    val String.foo : Any
+    get() {
+        this@f<caret>oo
+    }
+}

analysis/analysis-api/testData/referenceResolve/thisWithLabelToProperty_caretAtLabel.txt

@@ -0,0 +1,2 @@
+Resolved to:
+0: (in Foo) val kotlin.String.foo: kotlin.Any

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

@@ -0,0 +1,6 @@
+class Foo {
+    val String.foo : Any
+    get() {
+        t<caret>his@foo
+    }
+}

analysis/analysis-api/testData/referenceResolve/thisWithLabelToProperty_caretAtThis.txt

@@ -0,0 +1,2 @@
+Resolved to:
+0: extension receiver of val kotlin.String.foo: kotlin.Any