Code Quality: Hide unnecessary languages from GitHub Linguist (#1892)

* Hide C from Linguist

Whack!

* Hide Windows Command Processor from Linguist

Whack! Double points!

* Hide PowerShell from Linguist

THWACK!!! CRITICAL HIT!!
High score: 5 languages hit

* Bonus round: Hide Roff

Bonus round! 4 points for a total of 9 points! Well done, play again!

* Update .gitattributes to latest proposal

.gitattributes

@@ -45,5 +45,8 @@ tests/suite/file_without_BOM.txt -text
 # Do not normalize on commit.
 *.pdf -text
 
-*.pck linguist-detectable=false
-*.html linguist-documentation=true
+# Hide certain languages from GitHub Linguist
+* linguist-detectable=false
+*.cs linguist-detectable
+*.py linguist-detectable
+src/core/IronPython.StdLib/lib/** linguist-vendored