[ty] Improve rendering of default values for function args (#22010)

## Summary

We're actually quite good at computing this but the main issue is just
that we compute it at the type-level and so wrap it in `Literal[...]`.
So just special-case the rendering of these to omit `Literal[...]` and
fallback to `...` in cases where the thing we'll show is probably
useless (i.e. `x: str = str`).

Fixes https://github.com/astral-sh/ty/issues/1882

crates/ty_python_semantic/resources/mdtest/scopes/moduletype_attrs.md

@@ -97,7 +97,7 @@ inside the module:
 import typing
 
 reveal_type(typing.__name__)  # revealed: str
-reveal_type(typing.__init__)  # revealed: bound method ModuleType.__init__(name: str, doc: str | None = EllipsisType) -> None
+reveal_type(typing.__init__)  # revealed: bound method ModuleType.__init__(name: str, doc: str | None = ...) -> None
 
 # For a stub module, we don't know that `__file__` is a string (at runtime it may be entirely
 # unset, but we follow typeshed here):