Compile and use custom sysimages automatically

At package resolution time, also use `PackageCompiler` to generate a sysimage including all the packages in the environment, and load Julia with this sysimage.

Also provide a means for packages to specify precompile scripts to include in the sysimage.

Also have environment variables to disable this functionality, or disable certain packages from being included in the image.

Start at package resolution and investigate how PackageCompiler can generate and load a sysimage containing the environment's packages. Define how packages provide precompile scripts and how environment variables disable the feature or exclude packages; done means these controls and the automatic sysimage workflow are implemented and documented.

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