Fix super method call inside let expression (#1383)

Fixes #1309

The issue was that super calls were blocked inside let expressions
because:
1. The compiler's isClassMemberScope() check didn't skip over lambda
scopes created by let expressions
2. The runtime's findSupermethod() didn't traverse past VmFunction
owners to find the actual class prototype

Changes:
- SymbolTable.java: Updated isClassMemberScope() to skip lambda scopes
before checking if the parent is a class or module scope
- InvokeSuperMethodNode.java: Updated findSupermethod() to skip
VmFunction owners when looking for the class prototype

Added regression tests covering:
- Super method calls inside let expressions
- Super property access inside let expressions
- Nested let expressions with super calls

---------

Co-authored-by: Jen Basch <jbasch@apple.com>

pkl-core/src/test/files/LanguageSnippetTests/input/classes/supercallsInLet.pkl

@@ -0,0 +1,44 @@
+open class A {
+  function foo() = "a"
+}
+
+class B extends A {
+  function foo() =
+    let (bar = "b")
+      super.foo() + bar
+}
+
+local b = new B {}
+
+res1 = b.foo()
+
+// Also test with property access
+open class C {
+  value = "c"
+}
+
+class D extends C {
+  result =
+    let (x = "d")
+      super.value + x
+}
+
+local d = new D {}
+
+res2 = d.result
+
+// Test with nested let expressions
+open class E {
+  function getValue() = "e"
+}
+
+class F extends E {
+  function getValue() =
+    let (x = "f")
+      let (y = " and " + x)
+        super.getValue() + y
+}
+
+local f = new F {}
+
+res3 = f.getValue()

pkl-core/src/test/files/LanguageSnippetTests/output/classes/supercallsInLet.pcf

@@ -0,0 +1,3 @@
+res1 = "ab"
+res2 = "cd"
+res3 = "e and f"