add option to create ext when adding weakdeps

[IanButterworth]

Closes #3549

Creates

...
[weakdeps]
Chairmarks = "0ca39b1e-fe0b-4e98-acfc-b1656634c4de"
Example = "7876af07-990d-54b4-ab0e-23690620f79a"
ImageIO = "82e4d734-157c-48bb-816b-45c225c6df19"

[extensions]
ChairmarksImageIOExt = ["Chairmarks", "ImageIO"]
ExampleExt = ["Example"]

Note that this follows the naming guidance in #4000

TODO

• Tests
• Docs
• Also create ext/ExampleExt/ExampleExt.jl with boilerplate?

[KristofferC]

I'm bit sure this should be part of add. It feels more like a generate thing.

[IanButterworth]

I think there's elements of both. Not sure generate should add a weak dep, conversely.

It could be two steps:

pkg> add --weak Foo
pkg> generate --extension Foo

So those each are logical to add.

But for convenience I think offering to do the generate as part of add seems common enough to make sense.

Perhaps the default for noninteractive should be "n" though, so that would require two api calls.

[tecosaur]

Could --extension be made a flag of add instead? E.g.

pkg> add --weak Foo --ext FooExt