[ty] Add subdiagnostic suggestion to `unresolved-reference` diagnostic when variable exists on `self` (#18444)
## Summary
Closes https://github.com/astral-sh/ty/issues/502.
In the following example:
```py
class Foo:
x: int
def method(self):
y = x
```
The user may intended to use `y = self.x` in `method`.
This is now added as a subdiagnostic in the following form :
`info: An attribute with the same name as 'x' is defined, consider using
'self.x'`
## Test Plan
Added mdtest with snapshot diagnostics.
ruff
e658778c - [ty] Add subdiagnostic suggestion to `unresolved-reference` diagnostic when variable exists on `self` (#18444)
