refactor(spacing): generate spacing atomic classes programatically

[dancormier]

This PR expands on https://github.com/StackExchange/Stacks/pull/1652 and closes https://github.com/StackExchange/Stacks/issues/1653

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Spacing class generation is prone to bugs and accidental omissions. This PR make generation of spacing (and incidentally, positioning) atomic classes and documentation more programmatic.

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[giamir]

This refactor makes a lot of sense to me. I just glanced at the PR and left a comment. I will do a full review once you mark the PR as ready for review. Thanks @dancormier