Heuristic to auto-hide stdlib if too many

[tfiers]

stdlib is then not true/false, but rather: {:auto (default), :fade, :hide, :normal}

From

• Diff styling for stdlib & jll (faded) #47 (comment)

:auto chooses between :fade and :hide.
Based on some heuristic that avoids the eldritch Plots.jl horror of here:

• allow filtering out stdlibs and jlls #40

(we could keep false as an option, for backwards compat; and for consistency with jll, for which we provide no fade nor auto-hide)

[tfiers]

No, we should give jll's the same treatment:

• Colour jll's differently #83

Also, this def should go into the

• graph label #65:

"134 dependencies, of which 33 JLL packages and 28 stdlib packages were filtered out"

[tfiers]

Heuristic can just be a fixed number threshold :) Plots has about 66 (non stdlib) jll deps, eg. So cutoff could eg be 20.

[tfiers]

Btw, from

• graph label #65 (comment),

Whether and how many stdlib & jll packages hidden
No need to say "to unhide, pass ..": can say that in REPL :)
Btw, to give these numbers -- and to make the heuristic decision! -- we do need to crawl for them! (I.e. can't early stop in tree search in depgraph)

So there's diff between filtering them out post hoc, or not even crawling (expanding them)

So, the :hide option should split (or should stay, but mean sth more specific and there should be a new sibling option)

{:ignore, :hide, :fade, :normal, :auto (default)}

"
:auto chooses between the latter three:

• :normal if pkg is itself in the standard library
• :hide if not, and if there are more than hidethreshold() stdlib packages in the graph
• :fade otherwise.

I.e. we do show stdlib dependencies (but faded), if there aren't too many to clutter the graph.
"