SSMS has gone through quite a cycle since then. For example, with 2016 SSMS went to using the Visual Studio shell.
Have you, however, set up SSMS to treat .prc files as .sql files (as below)?
I just gave it a try and could treat a .prc file just like a .sql file. Opened it up in SSMS, could connect, and managed to execute SELECT @@VERSION; no problem. This, of course, doesn't change my opinion that you should save sql files as .sql files. 😉
Excuse my typos and sometimes awful grammar. My fingers work faster than my brain does.