Repro of for loop with context variable iterator variable

`let` bindings of context variables are lowered to a DeclareContext + 
StoreContext, which breaks codegen for `for` loops which expect that all 
statements of the init block will lower to variable declarations. The two 
instructions produce a variable declaration and a reassignment.

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/error.todo-for-loop-with-context-variable-iterator.expect.md

@@ -0,0 +1,35 @@
+
+## Input
+
+```javascript
+function Component() {
+  const data = useData();
+  const items = [];
+  // NOTE: `i` is a context variable because it's reassigned and also referenced
+  // within a closure, the `onClick` handler of each item
+  for (let i = MIN; i <= MAX; i += INCREMENT) {
+    items.push(<Stringify key={i} onClick={() => data.set(i)} />);
+  }
+  return items;
+}
+
+```
+
+
+## Error
+
+```
+   4 |   // NOTE: `i` is a context variable because it's reassigned and also referenced
+   5 |   // within a closure, the `onClick` handler of each item
+>  6 |   for (let i = MIN; i <= MAX; i += INCREMENT) {
+     |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>  7 |     items.push(<Stringify key={i} onClick={() => data.set(i)} />);
+     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>  8 |   }
+     | ^^^^ [ReactForget] Invariant: Expected a variable declaration (6:8)
+   9 |   return items;
+  10 | }
+  11 |
+```
+          
+      

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/error.todo-for-loop-with-context-variable-iterator.js

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+function Component() {
+  const data = useData();
+  const items = [];
+  // NOTE: `i` is a context variable because it's reassigned and also referenced
+  // within a closure, the `onClick` handler of each item
+  for (let i = MIN; i <= MAX; i += INCREMENT) {
+    items.push(<Stringify key={i} onClick={() => data.set(i)} />);
+  }
+  return items;
+}