Impl `Default` for `(String|Bytes|Boolean|None|Ellipsis)Literal` (#8341)
## Summary
This PR adds `Default` for the following literal nodes:
* `StringLiteral`
* `BytesLiteral`
* `BooleanLiteral`
* `NoneLiteral`
* `EllipsisLiteral`
The implementation creates the zero value of the respective literal
nodes in terms of the Python language.
## Test Plan
`cargo test`