AK+Everywhere: Remove StringView::find_{first,last}_of(char) methods

This removes StringView::find_first_of(char) and find_last_of(char) and
replaces all its usages with find and find_last respectively. This is
because those two methods are functionally equivalent.
find_{first,last}_of should only be used if searching for multiple
different characters, which is never the case with the char argument.

This also adds the [[nodiscard]] to the remaining find_{first,last}_of
methods.

Userland/Services/LaunchServer/Launcher.cpp

@@ -26,7 +26,7 @@ static bool spawn(String executable, const Vector<String>& arguments);
 
 String Handler::name_from_executable(const StringView& executable)
 {
-    auto separator = executable.find_last_of('/');
+    auto separator = executable.find_last('/');
     if (separator.has_value()) {
         auto start = separator.value() + 1;
         return executable.substring_view(start, executable.length() - start);