feat(preset-classic): guard against unknown keys in options

[Josh-Cena]

Motivation

Resolve #4239. After 8 months with no updates, let's do it ourselves

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Add an unknown key in the website's config and yarn start fails

Error stack

$ docusaurus start
Starting the development server...
Error: Unrecognized keys badOption found in preset-classic configuration. The allowed keys are debug, docs, blog, pages, sitemap, theme. Check the documentation: https://docusaurus.io/docs/presets#docusauruspreset-classic for more information.
    at preset (/Users/joshcena/Desktop/work/Tech/Projects/docusaurus/packages/docusaurus-preset-classic/lib/index.js:49:15)
    at /Users/joshcena/Desktop/work/Tech/Projects/docusaurus/packages/docusaurus/lib/server/presets/index.js:32:62
    at Array.forEach (<anonymous>)
    at loadPresets (/Users/joshcena/Desktop/work/Tech/Projects/docusaurus/packages/docusaurus/lib/server/presets/index.js:19:13)
    at loadPluginConfigs (/Users/joshcena/Desktop/work/Tech/Projects/docusaurus/packages/docusaurus/lib/server/index.js:85:84)
    at load (/Users/joshcena/Desktop/work/Tech/Projects/docusaurus/packages/docusaurus/lib/server/index.js:185:27)
    at async start (/Users/joshcena/Desktop/work/Tech/Projects/docusaurus/packages/docusaurus/lib/commands/start.js:40:19)
error Command failed with exit code 1.

[netlify]

✔️ [V2]

🔨 Explore the source changes: 34ba281

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/617cbe78210f7b0007aa0ba6

😎 Browse the preview: https://deploy-preview-5831--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟢 Performance	96
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5831--docusaurus-2.netlify.app/

[github-actions]

Size Change: -12 B (0%)

Total Size: 847 kB

ℹ️ View Unchanged

Filename	Size	Change
website/.docusaurus/globalData.json	38 kB	0 B
website/build/assets/css/styles.********.css	94 kB	0 B
website/build/assets/js/main.********.js	425 kB	0 B
website/build/blog/2017/12/14/introducing-docusaurus/index.html	68.2 kB	0 B
website/build/blog/index.html	39 kB	0 B
website/build/docs/index.html	46.1 kB	-6 B (0%)
website/build/docs/installation/index.html	53.6 kB	-6 B (0%)
website/build/index.html	31.6 kB	0 B
website/build/tests/docs/index.html	27.3 kB	0 B
website/build/tests/docs/standalone/index.html	23.8 kB	0 B

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[Josh-Cena]

Merging because we have more changes introduced in #5832 and we don't want two competing PRs.