obs-studio: add BROWSER option to enable CEF framework

OBS studio can now display a web browser in it's streaming setup..

• The BROWSER option works now: it depends on www/cef and points the build at it (CEF_ROOT_DIR=${LOCALBASE}/lib/cef). It is on by default on amd64 and not offered where www/cef does not build.
• Use the CMake switch obs-browser actually reads (ENABLE_BROWSER) and turn off the What's New dialog, which has no FreeBSD code path.
• obs-browser: FreeBSD support in its CMake files and linux code paths; load libcef.so from the CEF directory (RPATH) instead of copying CEF into lib/obs-plugins, and let CEF find its own locales; build with C++20 as the CEF 152 headers require; include <netinet/in.h> for sockaddr_in.

PR: 299341