Add .sc as a SuperCollider file extension

This pull request adds `.sc` as a SuperCollider file extension as requested at #1091.
It is the standard one for SuperCollider class files.
This extension is already used by Scala.

I tested it on several Scala and SuperCollider repositories.
It's not perfect but I guess it's better than before since it wasn't recognized at all.
- [scala-for-the-impatient](https://github.com/bodiam/scala-for-the-impatient.git) - Scala
```
95.41%  Scala
4.59%   SuperCollider
```
- [scala4impatientasks](https://github.com/adsinc/scala4impatientasks.git) - Scala
```
98.59%  Scala
1.41%   SuperCollider
```
- [scala-quiz](https://github.com/codingnirvana/scala-quiz.git) - Scala
```
91.09%  Scala
8.91%   Java
```
- [Scala-learning](https://github.com/iBuddha/Scala-learning.git) - Scala
```
91.43%  Scala
8.27%   SuperCollider
0.30%   Java
```
- [hack_listener](https://github.com/audiohackersalon/hack_listener.git) - SuperCollider
```
100.00% SuperCollider
```
- [scope](https://github.com/audiohackersalon/hack_listener.git) - SuperCollider
```
97.96%  JavaScript
2.04%   SuperCollider
```
- [SuperCute](https://github.com/danstowell/SuperCute.git) - SuperCollider
```
53.73%  C++
23.45%  SuperCollider
13.55%  C
2.03%   Python
1.82%   Emacs Lisp
1.75%   Objective-C++
1.57%   Objective-C
0.65%   Scala
0.58%   Perl
0.46%   VimL
0.22%   Ruby
0.10%   Shell
0.06%   Rebol
0.03%   R
```
- [Thr44-SC-quark](https://github.com/Thr44/Thr44-SC-quark.git) - SuperCollider
```
100.00% SuperCollider
```
- [mudrobot](https://github.com/fwolf/mudrobot.git) - SuperCollider
```
78.04%  SuperCollider
10.02%  C
8.85%   Scala
2.83%   PHP
0.26%   Shell
```