fix(core): configurePostCss() should run after configureWebpack()

[slorber]

Motivation

Fix #10106

It should be possible to have a plugin using configureWebpack() adding some kind of CSS loader, and then have a plugin calling configurePostCss() that configures postCSS for the newly added loader.

Test Plan

New unit tests added

The preview should keep working as before

Test links

https://deploy-preview-10132--docusaurus-2.netlify.app/

[netlify]

✅ [V2]

Name	Link
🔨 Latest commit	99705a3
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/66420786e0c13e0008a96b5e
😎 Deploy Preview	https://deploy-preview-10132--docusaurus-2.netlify.app
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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 65	🟢 98	🟢 96	🟢 100	🟠 88	Report
/docs/installation	🟠 55	🟢 96	🟢 100	🟢 100	🟠 88	Report
/docs/category/getting-started	🟠 76	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog	🟠 69	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 64	🟢 96	🟢 100	🟢 100	🟠 88	Report
/blog/tags/release	🟠 68	🟢 100	🟢 100	🟠 80	🟠 88	Report
/blog/tags	🟠 74	🟢 100	🟢 100	🟢 90	🟠 88	Report

[github-actions]

Size Change: 0 B

Total Size: 1.71 MB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/codeTranslations.json	2 B
website/.docusaurus/docusaurus.config.mjs	26.9 kB
website/.docusaurus/globalData.json	107 kB
website/.docusaurus/i18n.json	930 B
website/.docusaurus/registry.js	275 kB
website/.docusaurus/routes.js	179 kB
website/.docusaurus/routesChunkNames.json	119 kB
website/.docusaurus/site-metadata.json	2.17 kB
website/build/assets/css/styles.********.css	112 kB
website/build/assets/js/main.********.js	853 kB
website/build/index.html	38.1 kB

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Build	Status	Details	Updated (UTC)
default (Inspect)	✅ No change detected	-	May 13, 2024, 12:36 PM