lazy loading of the JET integration

[aviatesk]

JET sometimes ends up being incompatible with the latest version of Julia, and it can also prevent SnoopCompile.jl from loading. To get around this, this commit makes the JET integration lazy-loaded, so that it does not prevent SnoopCompile.jl from loading.

[aviatesk]

@timholy I'd like to push this PR through quickly and I'm inclined to go ahead and merge it. However I'll hold off on putting out a new version just yet. If you have any feedback, please drop a comment. Otherwise, I'll look to release a new version when it feels appropriate.

[mkitti]

Could you fix the tests?

[aviatesk]

I didn't dig into the failure on the stable version. We should investigate it, but let's do that in a separate PR.

[timholy]

Thanks for doing this! FYI, given infinite time I'd be slimming down the number of Julia versions supported and the overall functionality of this package to probably just @snoopr, @snoopi_deep, and @snoopl. In other words, I'd support any efforts that aim in that direction, given suitable semver caretaking which you know all about.

[timholy]

#382 (comment) pointed out that perhaps we should still have a [compat] entry for JET?