We actually don't want callers to have control over the lifetime of the
profiler object. It should be tied entirely to the GjsContext. This makes
most of the profiler API private.

Now, to control the profiler, you have two construct-only properties of
GjsContext: profiler-enabled and profiler-sigusr2. profiler-sigusr2
implies profiler-enabled, and does the work that
gjs_profiler_setup_signals() would previously have accomplished. The
GJS_PROFILER_ENABLED environment variable, if set, overrides the value of
profiler-signal.

By default when you call gjs_context_eval(), the profiler will be started
and stopped automatically around the evaluation of the JS code. If you
need to call gjs_context_eval() more than once, you probably only want to
do that within one profiling session. For that case we provide the
gjs_profiler_start() and gjs_profiler_stop() API, which will prevent the
auto-starting and stopping.