Highlighting for user-defined functions

[itsdfish]

Currently, user-defined functions and variables have the same highlight coloring. It would improve the readability of code if the user-defined and Stan functions had the same color.

[jrnold]

I'm going to go back through and try to standardize the scope names in the grammar. However, it may not directly solve this issue. I'm choosing the scopes names based on semantics, and to resemble other grammars. This may or may not end up producing reasonable colors for any given color scheme. It may require some customization of color themes to get them to work reasonably with Stan.

[itsdfish]

Thank you for your willingness to look into the issue.