Add support for Rust edition 2021

[KokaKiwi]

As in rust-lang/cargo#8922, the field package.edition in Cargo.toml can now be set to "2021"

The cargo.toml.json schema is then outdated as of now:

taplo/schemas/cargo.toml.json

Lines 417 to 427 in 7733a93

	"Edition": {
	"title": "Edition",
	"description": "The `edition` key affects which edition your package is compiled with. Cargo\nwill always generate packages via [`cargo new`](https://doc.rust-lang.org/cargo/commands/cargo-new.html) with the `edition` key set to the\nlatest edition. Setting the `edition` key in `[package]` will affect all\ntargets/crates in the package, including test suites, benchmarks, binaries,\nexamples, etc.",
	"type": "string",
	"enum": ["2015", "2018"],
	"x-taplo": {
	"links": {
	"key": "https://doc.rust-lang.org/stable/edition-guide/introduction.html"
	}
	}
	},