Switch eng/common to linguist-vendored (#6022)

This is a more accurate description of what's going on here;
the code canonically lives in dotnet/arcade and so is vendored
here. Additionally marking as generated so that it's hidden
from diffs by default (you can still click to expand if you're
curious).

.gitattributes

@@ -20,7 +20,7 @@ src/Tasks/*.targets linguist-detectable=true
 src/Tasks/*.tasks linguist-detectable=true
 
 # Don't include Arcade-owned path
-eng/common/** linguist-detectable=false
+eng/common/** linguist-vendored linguist-generated
 
 # Display XLF files collapsed by default in PR diffs
 *.xlf linguist-generated=true