script: expose theme_register to python, perl, ruby, lua, tcl, javascript, php, guile

Add weechat.theme_register (name, overrides) to all eight script
languages. Each binding is a mechanical translation of the same
signature:

  - name:      string
  - overrides: language-native dict / hash / associative array of
               full_option_name -> value strings
  - returns:   pointer-as-string of the registered t_theme (empty
               string on failure)

Each binding converts the dict to a struct t_hashtable using the
existing per-language helper (weechat_python_dict_to_hashtable,
weechat_perl_hash_to_hashtable, weechat_ruby_hash_to_hashtable,
weechat_lua_tohashtable, weechat_tcl_dict_to_hashtable,
weechat_js_object_to_hashtable, weechat_php_array_to_hashtable,
weechat_guile_alist_to_hashtable), calls weechat_theme_register,
frees the temporary hashtable, and returns the result. The new
function is registered right after the config_* functions so the API
listing stays grouped by topic.

PHP also receives a new arginfo entry (string, array -> string) in
both weechat-php_arginfo.h and weechat-php_legacy_arginfo.h.

This is plumbing only - the underlying theme_register function is
already covered by tests/unit/core/test-core-theme.cpp
(TEST(CoreTheme, Register)). No script-side tests are added here.

src/plugins/python/weechat.pyi

@@ -1143,6 +1143,19 @@ def config_unset_plugin(option_name: str) -> int:
     ...
 
 
+def theme_register(name: str, overrides: Dict[str, str]) -> str:
+    """`theme_register in WeeChat plugin API reference <https://weechat.org/doc/weechat/api/#_theme_register>`_
+    ::
+
+        # example
+        weechat.theme_register("light", {
+            "irc.color.input_nick": "cyan",
+            "irc.color.topic_old": "darkgray",
+        })
+    """
+    ...
+
+
 def key_bind(context: str, keys: Dict[str, str]) -> int:
     """`key_bind in WeeChat plugin API reference <https://weechat.org/doc/weechat/api/#_key_bind>`_
     ::