TODO: Add Interfaces - ScriptClient and others

[AlbertShown]

To make other wrappers able to use webui_xxx_client() APIs, we should add those new interfaces:

// Original C APIs:
//
// webui_show_client(webui_event_t* e, const char* content);
// webui_close_client(webui_event_t* e);
// webui_send_raw_client(webui_event_t* e, const char* function, const void* raw, size_t size);
// webui_navigate_client(webui_event_t* e, const char* url);
// webui_run_client(webui_event_t* e, const char* script);
// webui_script_client(webui_event_t* e, const char* script, size_t timeout, char* buffer, size_t buffer_length);

// New equivalent interfaces:
webui_interface_show_client(size_t window, size_t event_number, const char* content);
webui_interface_close_client(wsize_t window, size_t event_number);
webui_interface_send_raw_client(size_t window, size_t event_number, const char* function, const void* raw, size_t size);
webui_interface_navigate_client(size_t window, size_t event_number, const char* url);
webui_interface_run_client(size_t window, size_t event_number, const char* script);
webui_interface_script_client(size_t window, size_t event_number, const char* script, size_t timeout, char* buffer, size_t buffer_length);

[AlbertShown]

Needed by webui-dev/deno-webui#66.

[AlbertShown]

Added 0ff3b13.