[red-knot] Remove Type::Unbound (#13980)

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## Summary

- Remove `Type::Unbound`
- Handle (potential) unboundness as a concept orthogonal to the type
system (see new `Symbol` type)
- Improve existing and add new diagnostics related to (potential)
unboundness

closes #13671 

## Test Plan

- Update existing markdown-based tests
- Add new tests for added/modified functionality

crates/red_knot_python_semantic/src/lib.rs

@@ -20,6 +20,7 @@ pub mod semantic_index;
 mod semantic_model;
 pub(crate) mod site_packages;
 mod stdlib;
+pub(crate) mod symbol;
 pub mod types;
 mod util;