AK: Use an enum instead of a bool for String::replace(all_occurences)

This commit has no behavior changes.

In particular, this does not fix any of the wrong uses of the previous
default parameter (which used to be 'false', meaning "only replace the
first occurence in the string"). It simply replaces the default uses by
String::replace(..., ReplaceMode::FirstOnly), leaving them incorrect.

Userland/Libraries/LibWeb/URL/URLSearchParams.cpp

@@ -54,7 +54,7 @@ Vector<QueryParam> url_decode(StringView input)
         }
 
         // 4. Replace any 0x2B (+) in name and value with 0x20 (SP).
-        auto space_decoded_name = name.replace("+"sv, " "sv, true);
+        auto space_decoded_name = name.replace("+"sv, " "sv, ReplaceMode::All);
 
         // 5. Let nameString and valueString be the result of running UTF-8 decode without BOM on the percent-decoding of name and value, respectively.
         auto name_string = AK::URL::percent_decode(space_decoded_name);