feature request: measure compilation time

[Krastanov]

The result of using Pkg; @time @eval Pkg.precompile("PackageName") with an empty precompile cache. The empty cache can be ensured with modifying the JULIA_DEPOT environmental variable.

We would also need to modify ENV["JULIA_PKG_PRECOMPILE_AUTO"]=0. And might want to have access the number of precompilation tasks: ENV["JULIA_NUM_PRECOMPILE_TASKS"] which became available in 1.9.

[MilesCranmer]

With precompilation time getting even slower on Julia 1.10, I feel like this is badly needed! Some of my libraries seem to have compiled much slowly lately...