add notice of excessive scheduling time to time macro etc.

[IanButterworth]

Replaces #55094

Balancing number of tasks vs. task duration is a key optimization aspect, so this aims to bring a warning when it's notably imbalanced to the most common place that a user might try to see why their function is taking so long etc.

If more than 10% of wall time is spent (on average across all worker threads) specifically scheduling tasks then warn in @time.

julia> function foo(n; spawn=true)
          for i in 1:n
              if spawn
                  fetch(Threads.@spawn global j = i)
              else
                  global j = i
              end
           end
       end
foo (generic function with 4 methods)

julia> @time foo(1000)
  0.018877 seconds (10.71 k allocations: 396.266 KiB, 99.81% scheduling time, 11.34% compilation time)

julia> @time foo(1000)
  0.017071 seconds (9.97 k allocations: 358.875 KiB, 99.71% scheduling time)

julia> @time foo(1000; spawn=false)
  0.000004 seconds (489 allocations: 7.641 KiB)

julia> @time foo(1000; spawn=false)
  0.000004 seconds (489 allocations: 7.641 KiB)

A real world example that doesn't show the schedule warning.

julia> @time Pkg.update()
    Updating registry at `~/.julia/registries/General.toml`
   Installed JpegTurbo_jll ── v3.0.4+0
   Installed LZO_jll ──────── v2.10.2+1
   Installed Zstd_jll ─────── v1.5.6+1
   Installed Glib_jll ─────── v2.80.5+0
   Installed GtkObservables ─ v2.1.3
   Installed Revise ───────── v3.6.0
  Installing artifacts  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 4/4
    Updating `~/.julia/environments/v1.12/Project.toml`
  [295af30f] ↑ Revise v3.5.18 ⇒ v3.6.0
    Updating `~/.julia/environments/v1.12/Manifest.toml`
  [8710efd8] ↑ GtkObservables v2.1.2 ⇒ v2.1.3
  [295af30f] ↑ Revise v3.5.18 ⇒ v3.6.0
  [7746bdde] ↑ Glib_jll v2.80.2+0 ⇒ v2.80.5+0
  [aacddb02] ↑ JpegTurbo_jll v3.0.3+0 ⇒ v3.0.4+0
  [dd4b983a] ↑ LZO_jll v2.10.2+0 ⇒ v2.10.2+1
  [3161d3a3] ↑ Zstd_jll v1.5.6+0 ⇒ v1.5.6+1
Precompiling packages finished.
  97 dependencies successfully precompiled in 37 seconds. 34 already precompiled.
 44.095062 seconds (18.75 M allocations: 1.120 GiB, 0.56% gc time, 71 lock conflicts, 8.85% compilation time: 18% of which was recompilation)

[IanButterworth]

Updated the examples in the OP. With the new more precise timing a complex Pkg.update doesn't trigger the report.

[IanButterworth]

It'd be good to get this in to try it out on wild code to see if it reports meaningfully and hone the threshold if necessary.

But it'd be good to get a review for the timing measurement logic by someone familiar with the scheduler first.