chore: add linting config

[sampotts]

As discussed, this adds an ESLint config using Antfu's config.

Obviously we can tweak rules as we go but I find the defaults a pretty much "best practice".

Initial run of the lint task came up with a few things to fix:

✖ 628 problems (537 errors, 91 warnings)
  465 errors and 1 warning potentially fixable with the `--fix` option.

Enabling it

I've not enabled any automatic stuff yet but I guess we'll want to at a suitable time. This would require changing .vscode/settings.json from:

{
  // At some point, we'll watch to flick this on and uncomment lines 5 & 6 below.
  "eslint.enable": false,
  // Disable the default formatter, use eslint instead
  // "prettier.enable": false,
  // "editor.formatOnSave": false,
}

to:

{
  // Disable the default formatter, use eslint instead
  "prettier.enable": false,
  "editor.formatOnSave": false,
}

and in package.json for the git hooks, by uncommenting:

• the prepare script (which will add the git hooks)
• the simple-git-hooks section (which allows us to hook into a commit)
• the lint-staged section (which allows fixing issues at the staging step of the commit).