Move has_header_token() next to the other header field helpers

It was defined as a static inline above the border line so that the split
build would not turn it into an exported symbol of the shared library, which
kept abidiff from reporting an added function. That put an internal helper's
location at the mercy of a CI check rather than of where it belongs: it reads
a header field the same way get_header_value() and get_combined_header_value()
do, and it is the closest sibling of the latter, both being about a list-valued
field spread over several field lines.

Define it as a plain inline beside them and forward-declare it with the
split() family it calls. Adding a symbol is a source and binary compatible
change, so let abidiff report it.

httplib.h

@@ -3503,28 +3503,8 @@ void split(const char *b, const char *e, char d, size_t m,
 bool split_find(const char *b, const char *e, char d,
                 std::function<bool(const char *, const char *)> fn);
 
-// Defined here rather than beside the other header-field helpers because both
-// call sites sit in the implementation section below. Kept static so that an
-// internal helper does not become an exported symbol of the shared library.
-static inline bool has_header_token(const Headers &headers,
-                                    const std::string &key,
-                                    const std::string &token) {
-  // RFC 9110 7.6.1: a comma-separated token list field such as Connection may
-  // carry several tokens, and RFC 9110 5.3 lets that list be split across
-  // several lines. Match complete tokens rather than searching the raw value,
-  // so that a value such as "notupgrade" is not read as the token "upgrade".
-  auto rng = headers.equal_range(key);
-  for (auto it = rng.first; it != rng.second; ++it) {
-    const auto &value = it->second;
-    if (split_find(value.data(), value.data() + value.size(), ',',
-                   [&](const char *b, const char *e) {
-                     return case_ignore::equal(std::string(b, e), token);
-                   })) {
-      return true;
-    }
-  }
-  return false;
-}
+bool has_header_token(const Headers &headers, const std::string &key,
+                      const std::string &token);
 
 std::string websocket_accept_key(const std::string &client_key);
 
@@ -7790,6 +7770,25 @@ inline std::string get_combined_header_value(const Headers &headers,
   return combined;
 }
 
+inline bool has_header_token(const Headers &headers, const std::string &key,
+                             const std::string &token) {
+  // RFC 9110 7.6.1: a comma-separated token list field such as Connection may
+  // carry several tokens, and RFC 9110 5.3 lets that list be split across
+  // several lines. Match complete tokens rather than searching the raw value,
+  // so that a value such as "notupgrade" is not read as the token "upgrade".
+  auto rng = headers.equal_range(key);
+  for (auto it = rng.first; it != rng.second; ++it) {
+    const auto &value = it->second;
+    if (split_find(value.data(), value.data() + value.size(), ',',
+                   [&](const char *b, const char *e) {
+                     return case_ignore::equal(std::string(b, e), token);
+                   })) {
+      return true;
+    }
+  }
+  return false;
+}
+
 template <typename Map>
 inline typename Map::mapped_type
 get_multimap_value(const Map &m, const std::string &key, size_t id) {