Patch: Closure Fix & Expression Returns

• Fixed broken import in __main__.py (skizze_cli → skizze.skizze_cli)

• Fixed closure bug where SkizzeFnNode was mutated at runtime to store closure_env, causing incorrect behavior when functions are defined multiple times or in nested scopes. Functions are now wrapped in a SkizzeFnValue runtime object that is separate from the AST node.

• if/else and while now return their evaluated value instead of discarding it