Support VS Code's foldable `#region` by adding `folding.markers` to `language-configuration.json`

**Is your feature request related to a problem? Please describe.**

In other languages like Python, putting `#region` and `#endregion` blocks in code results in VS Code's native foldable regions - that is, the code enclosed in the region can be collapsed (folded / hidden).

**Describe the solution you'd like**

Add `folding.markers` to `langauge-configuration.json`, as shown in VS Code docs: https://code.visualstudio.com/api/language-extensions/language-configuration-guide#folding