fix(v2): add missing 'react' and 'webpack' peer dependencies

[ylemkimon]

Motivation

• mdx-loader, plugin-google-analytics, plugin-google-gtag, and docusaurus-plugin-sitemap depends on @docusaurus/core, which peer-depends on react and react-dom (@mdx-js/react peer-depends on react@^16.13.1)
• mdx-loader depends on file-loader and url-loader, which peer-depends on webpack
  (Write your motivation here.)

so they should be listed as peer dependencies.

Note other packages also list them as peer dependencies:

docusaurus/packages/docusaurus-plugin-debug/package.json

Lines 25 to 28 in 3242549

	"peerDependencies": {
	"react": "^16.8.4",
	"react-dom": "^16.8.4"
	},

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Check following warnings are not shown in Yarn 2 E2E test (https://github.com/facebook/docusaurus/pull/3675/checks?check_run_id=1343372025#step:6:308):

  ➤ YN0002: │ @docusaurus/plugin-google-analytics@npm:2.0.0-alpha.66.NEW doesn't provide react-dom@^16.8.4 requested by @docusaurus/core@npm:2.0.0-alpha.66.NEW
  ➤ YN0002: │ @docusaurus/plugin-google-analytics@npm:2.0.0-alpha.66.NEW doesn't provide react@^16.8.4 requested by @docusaurus/core@npm:2.0.0-alpha.66.NEW
  ➤ YN0002: │ @docusaurus/plugin-google-gtag@npm:2.0.0-alpha.66.NEW doesn't provide react-dom@^16.8.4 requested by @docusaurus/core@npm:2.0.0-alpha.66.NEW
  ➤ YN0002: │ @docusaurus/plugin-google-gtag@npm:2.0.0-alpha.66.NEW doesn't provide react@^16.8.4 requested by @docusaurus/core@npm:2.0.0-alpha.66.NEW
  ➤ YN0002: │ @docusaurus/plugin-sitemap@npm:2.0.0-alpha.66.NEW doesn't provide react-dom@^16.8.4 requested by @docusaurus/core@npm:2.0.0-alpha.66.NEW
  ➤ YN0002: │ @docusaurus/plugin-sitemap@npm:2.0.0-alpha.66.NEW doesn't provide react@^16.8.4 requested by @docusaurus/core@npm:2.0.0-alpha.66.NEW
  ➤ YN0002: │ @docusaurus/mdx-loader@npm:2.0.0-alpha.66.NEW doesn't provide react-dom@^16.8.4 requested by @docusaurus/core@npm:2.0.0-alpha.66.NEW
  ➤ YN0002: │ @docusaurus/mdx-loader@npm:2.0.0-alpha.66.NEW doesn't provide react@^16.8.4 requested by @docusaurus/core@npm:2.0.0-alpha.66.NEW
  ➤ YN0002: │ @docusaurus/mdx-loader@npm:2.0.0-alpha.66.NEW doesn't provide react@^16.13.1 requested by @mdx-js/react@npm:1.6.19
  ➤ YN0002: │ @docusaurus/mdx-loader@npm:2.0.0-alpha.66.NEW doesn't provide webpack@^4.0.0 || ^5.0.0 requested by file-loader@npm:6.2.0
  ➤ YN0002: │ @docusaurus/mdx-loader@npm:2.0.0-alpha.66.NEW doesn't provide webpack@^4.0.0 || ^5.0.0 requested by url-loader@npm:4.1.1

Related PRs

[docusaurus-bot]

Deploy preview for docusaurus-2 ready!

Built without sensitive environment variables with commit 0f10713

https://deploy-preview-3675--docusaurus-2.netlify.app

[slorber]

thanks