Add taplo configuration file to format TOML files

[dtolnay]

Taplo is the TOML formatter used by VS Code's TOML extension. https://taplo.tamasfe.dev

I have not added a GitHub Actions job to enforce formatting because it isn't necessarily valuable enough to be something every contributor to this repo needs to install, but I am open to it. Even without a CI job, someone manually running the formatter when things seem to be getting out of shape (a few years apart) would still be positive.

I have not touched the repos directory because I know there is a lot of work in flight by @Kobzol there. It can be formatted afterward. Edit: the formatter-generated changes in that directory are minor.

[rylev]

I'm personally in favor of this because my editor already does this.

[weihanglo]

Not really a blocker. Just wonder what would happen when cargo fmt starts formatting Cargo.toml?

rust-lang/style-team#188

[dtolnay]

If the formatting clashes, we would add exclude = [..., "**/Cargo.toml"] in .taplo.toml.

[jackh726]

I think this is a good chance. Semi-minor nit though @dtolnay can you squash to two commits? One with the configuration, and one with the actual formatting changes? (Or, just one commit is fine really)

[dtolnay]

Squashed to 2 commits. The first commit is handwritten (only touching .taplo.toml) and the second commit is 100% generated by running taplo format in the repo root.