instrument fish to track and report metrics on its behavior

While reviewing the fix for issue #2743 it occurred to me we really have no idea if the default `pcre2_substitute()` output buffer size is optimal in _src/builtin_string.cpp_. And we can't know unless we instrument the code to keep track of how many times the initial buffer was big enough, we had to double it once, twice, etc. There are undoubtedly other parts of the code with similar behavior that we should be tracking and reporting.

To be determined is how to extract that information. A new builtin command? Dump it if a specific signal (e.g., SIGUSR1) is received? Something else? The one thing I'm certain of is this should not be tied to exclusively to the profiling facility. We really want these metrics to be tracked long term for interactive shells. For that matter it might be useful to allow dumping the profiling data on demand in order to get a better idea of what's going on with interactive shells (as opposed to individual shell scripts).