Add support for code folding

[kolovos]

TextMate language definitions support code folding rules. For example, if we wanted to fold content within curly brackets and multi-line comments, we could add the following rules:

<key>foldingStartMarker</key>
<string>\{\s*$|/\*\*|\[\*</string>
<key>foldingStopMarker</key>
<string>^\s*\}|^\s*\*\*/|^\s*\*\]</string>

• IntelliJ doesn't support code folding rules at the moment so these would be mainly useful for TextMate users
• We need to find out if these folding marker definitions can be reused across languages so that we don't have to copy and paste them acroll all our language definitions