allow ^ in URLs

Users may select specific outputs using the ^output syntax or selecting
any output using ^*.

URL parsing currently doesn't support these kinds of output references:
parsing will fail.

Currently `queryRegex` was reused for URL fragments, which didn't
include support for ^. Now queryRegex has been split from fragmentRegex,
where only the fragmentRegex supports ^.

src/libexpr/flake/flakeref.cc

@@ -190,7 +190,7 @@ std::optional<std::pair<FlakeRef, std::string>> parseFlakeIdRef(
 
     static std::regex flakeRegex(
         "((" + flakeIdRegexS + ")(?:/(?:" + refAndOrRevRegex + "))?)"
-        + "(?:#(" + queryRegex + "))?",
+        + "(?:#(" + fragmentRegex + "))?",
         std::regex::ECMAScript);
 
     if (std::regex_match(url, match, flakeRegex)) {