chore(v2): Update webpack-related dependencies, pre-work to migrate to Webpack 5

[RDIL]

Motivation

Begin to implement #4027 by updating the webpack plugins & related code.

Notes for webpack 5 upgrade:

• mini-css-extract-plugin on latest BREAKS SEARCH stuff in the production build
• html-webpack-plugin will need an update
• null-loader should be removed
• terser-webpack-plugin on latest just doesn't work

Have you read the Contributing Guidelines on pull requests?

Yes.

Test Plan

I've confirmed it still works just fine on the current webpack version (v4).

Related PRs

n/a

[netlify]

[V1] Deploy preview failure

Built without sensitive environment variables with commit 1387533

https://app.netlify.com/sites/docusaurus-1/deploys/6008d1b36e050d0008ab5fc8

[netlify]

Deploy preview for docusaurus-2 ready!

Built without sensitive environment variables with commit 1387533

https://deploy-preview-4081--docusaurus-2.netlify.app

[github-actions]

Size Change: -2 B (0%)

Total Size: 27.1 kB

ℹ️ View Unchanged

Filename	Size	Change
website/build/blog/2017/12/14/introducing-docusaurus/index.html	20.9 kB	-2 B (0%)
website/build/docs/introduction/index.html	180 B	0 B
website/build/index.html	6.02 kB	0 B

_{compressed-size-action}

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟢 Performance	90
🟢 Accessibility	99
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-4081--docusaurus-2.netlify.app/classic/

[semoal]

About terser: If you are using webpack v5 or above you do not need to install this plugin. Webpack v5 comes with the latest terser-webpack-plugin out of the box. Using Webpack v4

About html-webpack-plugin: actually you have to install @next version to make it compatible with webpack 5

[RDIL]

About terser: If you are using webpack v5 or above you do not need to install this plugin. Webpack v5 comes with the latest terser-webpack-plugin out of the box. Using Webpack v4

About html-webpack-plugin: actually you have to install @next version to make it compatible with webpack 5

Yes, but this PR is just to bump up the non-critical dependencies to make the actual webpack 5 PR smaller.

[slorber]

Ah I see, that's why the deploy preview looks fine to me 😅

Thanks for your work, will review it soon

[slorber]

LGTM didn't see any issue