feat(core): Add ability to set stack description (#4457)

[TomDufall]

Add the ability to set a description for a stack.
Closes #4457

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

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[TomDufall]

A few questions to resolve:

• I'm not sure how docs work - will this be automatically picked up or do I need to add it somewhere?
• I haven't tried actually making a stack manually since I'm not sure how to do that with a local version of the cdk. If someone more knowledgeable could either do this or verify that I'm definitely testing stack output correctly, that would be great
• The CloudFormation length limit for the description is listed as 1024 bytes here. A quick Google suggests this'll probably be 512 characters for most uses, so that's what I've tested for. Any clarification or better test method is welcome
• This is the first TS I've written, so watch out for beginner or convention errors. It compiles locally and passes the linter, though, so should be fine.

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