`findSkip` can now skip elements using just a predicate function

Previously, $(REF findSkip, std, algorithm, searching) could only
be used to find a specific string. Now, a new overload allows elements
in a range to be skipped over if the passed function returns `true`:

-------
import std.ascii : isWhite;
string s = "   abc";
assert(findSkip!isWhite(s) == 3 && s == "abc");
assert(!findSkip!isWhite(s) && s == "abc");
-------
