fix: report tail-position return-value mismatches

[ivov]

Tail-position type mismatches in fn and lambda bodies are now reported as errors, not silently discarded, matching Rust semantics and preventing invalid Go output for cases like fn test() { "test".length() }

fn test() {
  42
}

[error] Mismatch between return type and return value
  ╭─[test.lis:3:3]
1 │
2 │ fn test() {
  ·           ┬
  ·           ╰── has `()` as implicit return type
3 │   42
  ·   ─┬
  ·    ╰── returns `int`
4 │ }
  ╰────
help: If the `()` return type is intended, discard the return value with `let _ = ...`. 
If the `int` return value is intended, add `-> int` to the function signature · code:
[lint.mismatched_return_value]

Fix #252

[github-actions]

Released in lisette-v0.1.24