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/Applications/Spreadsheet/Spreadsheet.cpp

@@ -91,7 +91,7 @@ static size_t convert_from_string(StringView str, unsigned base = 26, StringView
 
     size_t value = 0;
     for (size_t i = str.length(); i > 0; --i) {
-        auto digit_value = map.find_first_of(str[i - 1]).value_or(0);
+        auto digit_value = map.find(str[i - 1]).value_or(0);
         // NOTE: Refer to the note in `String::bijective_base_from()'.
         if (i == str.length() && str.length() > 1)
             ++digit_value;