[cmake] Add ENABLE_WERROR option (#4080)

Should warnings be errors? that's a philosophical debate which I can only answer with "no, but if you think otherwise here is an option for you"

Signed-off-by: lizzie <lizzie@eden-emu.dev>

Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/4080
Reviewed-by: crueter <crueter@eden-emu.dev>
Reviewed-by: MaranBr <maranbr@eden-emu.dev>

src/core/CMakeLists.txt

@@ -1199,7 +1199,7 @@ if (MSVC)
     )
 else()
     target_compile_options(core PRIVATE
-        $<$<COMPILE_LANGUAGE:C,CXX>:-Werror=conversion>
+        $<$<COMPILE_LANGUAGE:C,CXX>:-Wconversion>
         $<$<COMPILE_LANGUAGE:C,CXX>:-Wno-sign-conversion>
         $<$<COMPILE_LANGUAGE:C,CXX>:-Wno-cast-function-type>
         $<$<CXX_COMPILER_ID:Clang>:-fsized-deallocation>)