Extend "markdown" features to more applicable pages #4244

As discovered in https://github.com/sindresorhus/refined-github/pull/4151, we have some features that are applied only to Markdown files but could be applied to a whole host of file types like CSV and SVG. Some other types can be found on https://github.com/github/markup/blob/46908504f5099b15a88030749f0aeb52cd2f6799/README.md#markups

Please analyze what makes a feature applicable to other filetypes and don't just blindly include random extensions.

Some of these features are:

• vertical-front-matter
• view-markdown-source
• hide-markdown-diff -> if compatible with other files, it needs to be reimplemented because it was added by GitHub… only on markdown files.
• collapse-markdown-sections should be changed to only apply to wikis, as discussed in https://github.com/sindresorhus/refined-github/issues/4008#issuecomment-820515283 done in #4335