feat(core): allow plugins to self-disable by returning null

[slorber]

Motivation

A plugin should be able to self-disable depending on context/options by returning null

We actually need this for our own plugins:

export default function pluginPWA(
  context: LoadContext,
  options: PluginOptions,
): Plugin<void> | null {
  if (process.env.NODE_ENV !== 'production') {
    return null;
  }
  if (context.siteConfig.future.experimental_router === 'hash') {
    logger.warn(
      `${PluginName} does not support the Hash Router and will be disabled.`,
    );
    return null;
  }
  
  return {
    name: PluginName, 
    // rest of plugin here...
  }
}

See also #10228 (comment)

Test Plan

unit tests

Test links

https://deploy-preview-10286--docusaurus-2.netlify.app/

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Build	Status	Details	Updated (UTC)
default (Inspect)	✅ No changes detected	-	Jul 10, 2024, 10:55 AM

[github-actions]

Size Change: 0 B

Total Size: 1.85 MB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/codeTranslations.json	2 B
website/.docusaurus/docusaurus.config.mjs	27.3 kB
website/.docusaurus/globalData.json	123 kB
website/.docusaurus/i18n.json	930 B
website/.docusaurus/registry.js	304 kB
website/.docusaurus/routes.js	202 kB
website/.docusaurus/routesChunkNames.json	130 kB
website/.docusaurus/site-metadata.json	2.17 kB
website/build/assets/css/styles.********.css	113 kB
website/build/assets/js/main.********.js	907 kB
website/build/index.html	38.1 kB

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