Make statement blocks proposal

[jonmeow]

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Indentation-based validity rules, particularly for code blocks (as in Python), were the topic of leads question https://github.com/carbon-language/carbon-lang/issues/521. The question was resolved, and the comment https://github.com/carbon-language/carbon-lang/issues/521#issuecomment-890268632 is the key decision.

Because the question was asked independent of a proposal, a standard proposal is still needed for evolution review, essentially translating the issue into a proposed solution with rationale, alternatives considered, etc. This issue is to track the proposal need.

https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/blocks_and_statements.md tracks the design status of blocks. Given that the design was not to do indentation-based validity rules, I think the right structure of this is a proposal laying out the structure of blocks and scopes, mentioning indentation-based rules as an alternative considered.

[chandlerc]

I feel like #2002 should maybe be merged into this one, what do others think?

[junlarsen]

I'm going to start work on #2002 and to me, it also makes sense to have a look at this at the same time.

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