implemented strictCaseObjects (#20608)

* implemented strictCaseObjects

* changelog update

tests/effects/tstrict_caseobjects.nim

@@ -0,0 +1,27 @@
+discard """
+  errormsg: "field access outside of valid case branch: x.x"
+  line: 25
+"""
+
+{.experimental: "strictCaseObjects".}
+
+type
+  Foo = object
+    case b: bool
+    of false:
+      s: string
+    of true:
+      x: int
+
+var x = Foo(b: true, x: 4)
+case x.b
+of true:
+  echo x.x
+of false:
+  echo "no"
+
+case x.b
+of false:
+  echo x.x
+of true:
+  echo "no"