Add `smol_str03` feature

[phsym]

This pull request adds support for the smol_str crate version 0.3 (smol_str03) as an optional dependency alongside the existing support for version 0.2 (smol_str02). The documentation, Cargo configuration, and test suite have all been updated to reflect and validate this new compatibility.

Closes #490

[phsym]

@GREsau Thank you for this wonderful library, it's very helpful. I need support for smol_str v0.3.4, so I added it as an optional feature like it has been done for smol_str v0.2. However, it appeared that smol_str 0.3.4 requires to change MSRV to rust 1.89.

As changing MSRV can be considered a breaking change, do you have any idea about how I should deal with this issue ?

[phsym]

Nevermind, I downgraded it to smol_str v0.3.2 which has no MSRV restriction

[sgoll]

I'd love to see this merged into a new release as I just stumbled over this myself (and I would have opened a PR myself if I hadn't found this one). Great job @phsym 👍

Cc @GREsau