[fix] ignore css-unused-selector rule if <style global>

[metonym]

Fixes #123

This PR detects if the style block contains the "global" attribute and filters "css-unused-selector" warnings if true.

The desired UX is to have the following code not emit an ESLint warning:

<!-- `h1` tags exist inside this component -->
<Component />

<style global>
  h1 { color: red }
</style>

I added the global-style sample to test this.

Run yarn test; no errors should be emitted from the global-style sample.

Without this modification, running yarn test on the global-style sample would return the following warnings (expected):

[
  {
    "ruleId": "css-unused-selector",
    "severity": 1,
    "message": "Unused CSS selector \"h1\"",
    "line": 2,
    "column": 3,
    "endLine": 2,
    "endColumn": 5
  }
]

[dummdidumm]

Agree with this change. It's support for a svelte-preprocess option, not "vanilla Svelte", but that doesn't make it less viable in my opinion.
If you change the formatting (tabs instead of spaces) we're good to go 👍

[metonym]

@dummdidumm Updated 🙏

[metonym]

@dummdidumm Anything else I should do for this PR?