Implement string.contains() per CEL spec section 6.2.5

## Summary

The CEL specification (section 6.2.5) defines `string.contains(string) -> bool` as a standard string function. Currently, calling `.contains()` on a `StringType` raises `CELEvalError: no such overload` in cel-python.

## Steps to reproduce

```python
import celpy

env = celpy.Environment()
ast = env.compile('"hello world".contains("world")')
prog = env.program(ast)
result = prog.evaluate({})
print(result) # Expected: true
```

## Actual behaviour

```
celpy.celenv.CELEvalError: no such overload
```

## Expected behaviour

`contains()` should return `true` when the substring is present and `false` otherwise, matching the Go and Java CEL implementations.

## Proposed fix

Add a `contains` method to `StringType` in `src/celpy/celtypes.py`:

```python
def contains(self, other: 'StringType') -> BoolType:
return BoolType(str(other) in str(self))
```

And register the overload in the function dispatch table so that `"x".contains("y")` is correctly dispatched.

## Related
- Issue #128 (user question about `.contains()` not working)
- CEL spec reference: https://github.com/google/cel-spec/blob/master/doc/langdef.md#standard-definitions