fix(plugin-netlify-cms): exclude cms.css from index.html

[erezrokah]

Description

During the migration from Gatsby v2 to v3, html-webpack-exclude-assets-plugin was commented out since it doesn't support webpack 5.

This PR uses html-webpack-skip-assets-plugin to restore the same functionality as before.

Without this fix cms.css is added to the CMS index.html, thus styles are applied globally (to the CMS itself).
With the fix styles are only applied to the CMS preview pane via

gatsby/packages/gatsby-plugin-netlify-cms/src/cms.js

Line 25 in 44257b6

CMS.registerPreviewStyle(`cms.css`)

Related Issues

Fixes #31909

[LekoArts]

Thanks! Fortunately the author already published your changes in 1.0.2 so you can change your code a bit and remove the TODO :)

[erezrokah]

Thanks! Fortunately the author already published your changes in 1.0.2 so you can change your code a bit and remove the TODO :)

Thanks for the quick review @LekoArts, fixed in d7c087b