Lsts frontend upgrades fsdkwklel

[andrew4328]

Describe your changes

Features:

• unify left-hand side syntax
• if let
• while let
• for let
• lhs = rhs

Issue ticket number and link

#2060

Checklist before requesting a review

• [ x ] I have performed an AI-assisted self-review of my code.

Can you review my pull request and provide some suggestions?
https://patch-diff.githubusercontent.com/raw/Lambda-Mountain-Compiler-Backend/lambda-mountain/pull/1926.diff

• [ x ] If it is a new feature, I have added thorough tests.
• [ x ] I agree to release these changes under the terms of the permissive MIT license (1).

1. https://github.com/andrew-johnson-4/lambda-mountain/blob/main/LICENSE