Support `.toml` file extension on `.cargo/config`

[anp]

Describe the problem you are trying to solve

To have syntax highlighting in VSCode I currently create a custom file association in my settings:

    "files.associations": {
        "**/.cargo/config": "toml"
    },

I'd love to not have to do this!

Describe the solution you'd like

Could cargo accept .cargo/config.toml as a path for local configuration?

Notes

There are many alternatives available, like making the Rust editor plugins aware of this association. However, cargo's usage of TOML elsewhere is identified by the file extension (consistency), and this change would presumably improve ergonomics in other places that do syntax highlighting, not just in editors (availability).

[oli-obk]

We should just change our docs to show config.toml everywhere and support both config and config.toml for backwards compat. Noone will be affected and everyone can update as they please.

[fluffysquirrels]

What do we do where both config and config.toml exist? How about issuing a warning and defaulting to config? My preference leans toward issuing an error if both exist, but this is a breaking change affecting people that already have both for some reason.

[anp]

I agree re: an error being a weird change for someone who had both paths available.

Defaulting to config on conflict sounds great to me, since it matches the behavior someone with both files would already have observed. Printing a warning also sounds great to me.