Improve variable detection in checkvariables.php

[flodolo]

/%([0-9]+\$){0,1}S/i

This also detects %s or %S.
pro: you can find more errors (example for cs).
neg: more false positives. Code now reports a mismatch between %S and %s, and %S and %1$S, and both are correct. The alternative is to put more logic in the class

/(?<!%[0-9])\$[a-z0-9\.]+\b/i

This now matches only "brandshortname" in "test brandshortname.". Negative lookbehind is used to avoid some false positive, example My%1$SFeeds-%2$S.opml.

[pascalchevrel]

I guess it's a good tradeoff