chore: add devcontainer

[tristan-f-r]

adds a devcontainer with Deno (for www) and Rust (for the repo itself) support.

(If a contributor agrees) I do also think that the perf tools should be added as well (as mentioned in README), but for now it's a bare container.

[CLAassistant]

All committers have signed the CLA.

[magurotuna]

Thanks for adding the useful configuration!
Could you add .devcontainer directory to the format target in tools/format.ts, so deno fmt will be applied to devcontainer.json?
Like this:

const p3 = await Deno.run({
  cmd: [
    "deno",
    "fmt",
    ...checkArgs,
    "tools",
    "benchmarks/benchmarks.ts",
    "www/pages",
    "docs/rules",
    "README.md",
+   ".devcontainer",
  ],
  stdin: "null",
}).status();

[magurotuna]

Thanks! Please also don't remember to run the formatter so the CI passes :)