Merge #6111

6111: Add assist for converting the base of integer literals. r=SomeoneToIgnore a=vlakreeh

This PR adds an assist similar to Intellij's [convert number to](https://i.imgur.com/JH6wstP.png). It also does a small refactor to [assists/src/tests.rs](https://github.com/rust-analyzer/rust-analyzer/blob/fc34403018079ea053f26d0a31b7517053c7dd8c/crates/assists/src/tests.rs) to add the ability to specify the resolved assist for a specific action within an assist group.

## Demo
![Demo of the assist in action](https://i.imgur.com/MBhdPFH.gif)

Co-authored-by: vlakreeh <zeb@zebulon.dev>

Author: bors[bot]