**Description:**
This patch applies the variable collapsing logic for the variables declared with `var` to the variables declared with `let`. This requires some patch about the behavior of visitors about `BlockStmt`, so this patch includes relevant changes.
**Related issue:**
- Closes https://github.com/swc-project/swc/issues/5696.
- We don't need a list of bindings to check if an identifier is unresolved.
- Checking if an identifier is unresolved is now one CPU instruction.
- Previously it was one hashmap operation.
- This PR also improves performance, by removing the hashmaps mentioned above.
