Add user created snippets

[theRealPadster]

📝 Provide a description of the new feature

It would be sweet to add user created snippets to marketplace, so it's easier to get them added in, and we don't need people to make a PR and stuff every time they want to add/update one.

This would work much the same as the themes and extensions tabs, but have a snippetcss key or something and use a snippet github topic.

➕ Additional Information

No response

[CharlieS1103]

Yeah wasn't this the original plan with the snippets idea using firebase but then we opted to just do a file instead

[theRealPadster]

I don't remember. I'm proposing we just use a manifest like all the rest

[theRealPadster]

Should we make the manifest have a snippetcss field that is the css string, or have it point to a css file, like the usercss one does? Pointing to a file would let people keep things cleaner if they have more than one snippet, and would allow people to use less or scss and build the css itself. But also might just be overkill for snippets.