Update Stan highlighting

This updates the syntax for the Stan language to account for a few recent changes:

• Arrays can now be defined like array[dims] type ident, so something formerly written as real[4] xs can now be array[5] real xs
• Complex numbers have been added to Stan. This adds the complex type, and the literal syntax 3.14i for imaginary literals.
• A few new keywords have been added, including the use of profile blocks for profiling.

I am unfortunately unable to run the tests in this repository. If a maintainer would be willing to do so for me I would greatly appreciate it. I have tested this XML on our documentation