add fixture error demonstrating the stdlib parts being removed

Author: zth

tests/build_tests/super_errors/expected/stdlib_removed_in_error.res.expected

@@ -0,0 +1,15 @@
+
+  We've found a bug for you!
+  /.../fixtures/stdlib_removed_in_error.res:3:12-42
+
+  1 │ type x = Stdlib.Promise.t<int>
+  2 │ 
+  3 │ let x: x = Stdlib.Promise.resolve("hello")
+
+  This has type: Promise.t<string> (defined as promise<string>)
+  But it's expected to have type: x (defined as promise<int>)
+  
+  The incompatible parts:
+    string vs int
+  
+  You can convert string to int with Belt.Int.fromString.

tests/build_tests/super_errors/fixtures/stdlib_removed_in_error.res

@@ -0,0 +1,3 @@
+type x = Stdlib.Promise.t<int>
+
+let x: x = Stdlib.Promise.resolve("hello")