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/Browser/CookieJar.cpp

@@ -142,7 +142,7 @@ String CookieJar::default_path(const URL& url)
         return "/";
 
     StringView uri_path_view = uri_path;
-    std::size_t last_separator = uri_path_view.find_last_of('/').value();
+    size_t last_separator = uri_path_view.find_last('/').value();
 
     // 3. If the uri-path contains no more than one %x2F ("/") character, output %x2F ("/") and skip the remaining step.
     if (last_separator == 0)

Userland/Applications/SoundPlayer/M3UParser.cpp

@@ -64,7 +64,7 @@ NonnullOwnPtr<Vector<M3UEntry>> M3UParser::parse(bool include_extended_info)
             if (line.starts_with('#') && has_exteded_info_tag) {
                 if (line.starts_with("#EXTINF")) {
                     auto data = line.substring_view(8);
-                    auto separator = data.find_first_of(',');
+                    auto separator = data.find(',');
                     VERIFY(separator.has_value());
                     auto seconds = data.substring_view(0, separator.value());
                     VERIFY(!seconds.is_whitespace() && !seconds.is_null() && !seconds.is_empty());

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;