Add support for GJS_TRACE_FD

These commits adds support for a new GJS_TRACE_FD when using the gjs or gjs-console binaries. Doing so, when combined with the upcoming Sysprof-3, will automatically integrate GJS profiling data with other callgraph samples and system data. This is much simpler than integrating with SIGUSR2 or connecting via D-Bus (although we can support that too).

We'll also be integrating similar features into libraries like GTK with GTK_TRACE_FD meaning that we can tie profiling data from each of these sources into a single capture. Furthermore, applications themselves can use SYSPROF_TRACE_FD for custom application tracing.