fix(core): make plugin lifecycles consistently bound to the plugin instance

[Josh-Cena]

Motivation

This should always work:

function plugin() {
  return {
    foo() { console.log("Bar"); }
    configureWebpack() { this.foo(); }
  };
}

Unfortunately, it doesn't work if configureWebpack is unbound from the plugin instance. We should always bind the lifecycle so plugin authors can write code in the most intuitive way.

Have you read the Contributing Guidelines on pull requests?

Yes

[netlify]

✔️ [V2]

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	54
🟢 Accessibility	100
🟢 Best practices	92
🟢 SEO	100
🟢 PWA	90

Lighthouse ran on https://deploy-preview-6850--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 791 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	49.9 kB
website/build/assets/css/styles.********.css	105 kB
website/build/assets/js/main.********.js	597 kB
website/build/index.html	38.4 kB

_{compressed-size-action}

[slorber]

👍

Not particularly fan of this binding but why not 🤷‍♂️

If we want to keep this behavior over time it's worth adding some tests