Upgrade nufmt dependencies / toolchain

nufmt has fallen behind the main Nushell repository, and some of its dependencies are now deprecated.

Sooner or later it'll have to:

• migrate to Rust Edition 2024
• update dependencies to match the main Nushell repository
• remove and replace deprecated components

Given the version gap, preserving compatibility with the current dependency set probably isn't worth the additional complexity.

I have roughly implemented the updates and would be happy to open a PR if this approach seems reasonable. LLMs should be very suitable to help with this job. I promise that I will carefully review the code line by line. But since the current nufmt itself still “eats code” (https://github.com/nushell/nushell/discussions/18558), I think it's not very meaningful to keep the upgraded nufmt behavior exactly the same as before. But I'll try to maintain consistency with the original design goals and will make it passes all the tests.

I expect the workload won't be too large, but it will almost definitely generate A LOT OF diff. I'll also use .gitattributes to help minimize some of them.

Start by comparing nufmt's Rust toolchain and dependencies with the main Nushell repository, then identify deprecated components and the changes needed for Rust Edition 2024. Review the proposed approach and use .gitattributes to manage the large diff. Run all tests; done means the updated dependencies and toolchain work without regressions and the formatter no longer fails its existing tests.