[browser][MT] legacy JS interop needs GC boundaries on main thread too

Calling legacy `mono_wasm_invoke_method_ref` on the main thread could lead to GC transition assert at `mono-threads-state-machine.c:807`

See https://github.com/dotnet/runtime/issues/89909#issuecomment-1665811281