Added scm alias to scheme

[matyklug18]

Changes

Added scm alias to scheme

Checklist

• Added markup tests, or they don't apply here because...
• Updated the changelog at CHANGES.md

Note: I have been unable to find information about the required formatting / additional changes, or if this kind of PR is welcome at all. I have done what is my best guess, and as such I apologize if I've made any mistakes.

[joshgoebel]

SCM is a variant of Scheme, yes?

[matyklug18]

scm is the file extension for scheme source files, eg. how Rust uses rs or JavaScript js. Since both of those are aliased, I thought that it might be good to add Scheme's file extension. Me and a few other people have ran into the issue of it missing, typing scm and realizing that it doesn't work.