[folding] Preview folded content on hover

Found no existing issues It would be cool to preview folded content without actual unfolding it, using peek For example, how it's done in WebStorm: <details><summary>small gif</summary><img src="https://user-images.githubusercontent.com/599439/49730695-38986d00-fc8a-11e8-88b7-c07fffb9b45b.gif" /></details> video: https://streamable.com/i8z3u I think it should be in core. Or a plugin which distributes with vscode. What do you think? refs for code: css: https://github.com/Microsoft/vscode/blob/cb66c137e0279693a8fcea3ed481d1d2db7512c7/src/vs/editor/contrib/folding/folding.css ts: https://github.com/Microsoft/vscode/blob/4e4e6f9b44dd9e6a53d1f08565c52b8ea9f61a5e/src/vs/editor/contrib/folding/foldingDecorations.ts