Introduce a rustfmt.toml file with nightly features

[alcuadrado]

This PR introduces a rustfmt.toml that uses some nightly features and makes the necessary setup changes for it to work.

The first 5 commits implement the necessary setup to use nightly exclusively for rustfmt, while still compiling with stable. This includes the devcontainer (you need to rebuild it), the ci, pre-commit hook, and vscode settings.

Then, each of the following commits enable a rustfmt feature, and automatically reformat the code.

The features enabled are:

1. import_granularity: Controls how the import statements are merged. In this case, it automatically merges every import from the same crate, so that there's only 1 import per crate.
2. wrap_comments: Automatically wrap comments.
3. group_imports: Groups imports in three groups (i.e. std, core, and alloc; external crates; and self, super, and crate).

Features 1 and 2 were discussed with @Wodann. Feature 3 looked interesting to me (we use an equivalent in TS), but I have no strong opinions about it.

Other interesting features:

• comment_width: So that there's less wrapping.
• use_small_heuristics: This controls how often rustfmt introduces newlines. I saw multiple projects set it to "Max", so that lines are longer. For example, rust-analyzer uses it.

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[alcuadrado]

• use_small_heuristics: This controls how often rustfmt introduces newlines. I saw multiple projects set it to "Max", so that lines are longer. For example, rust-analyzer uses it.

I recommend you enable it and look at some diffs. I don't particularly like it, but it may be my lack of experience.

[alcuadrado]

• comment_width: So that there's less wrapping.

I played with this for a bit, and setting it to 100 would preserve most comments unchanged, so I believe that was the intended width.

If we decide to set it, I can do it and rebase this PR.

[agostbiro]

Thanks @alcuadrado this is a great addition!

I tried out the use_small_heuristics = "Max" option and I have a weak preference against it, as I prefer to see chained methods in a vertical layout.

I noticed that we don't cache the Rust toolchain installations currently so I added a backlog issue for that: NomicFoundation/edr#221

[alcuadrado]

I tried out the use_small_heuristics = "Max" option and I have a weak preference against it, as I prefer to see chained methods in a vertical layout.

Yeah, same here.

[Wodann]

Let's not change the use_small_heuristics for the reasons mentioned above.

For, comment_width, the width of comments in the PR matches the max length of code lines, which looks good to me.

[alcuadrado]

Rebased against edr/main. I'll merge once the CI passes.