Add support for let chains

[c410-f3r]

See rust-lang/rust#88642.

EDIT @ytmimi: Also see rust-lang/style-team#169 for the discussion on how let-chains should be formatted. I encourage anyone in the community to share their thoughts on the styling rules in the linked RFC. The formatting logic cannot be implemented in rustfmt until the formatting rules have been decided.

[c410-f3r]

Closing in favor of #4955

[ytmimi]

Going to reopen this and use it as the tracking issue for let-chain support.

[c410-f3r]

I am housekeeping my dashboard closing issues that are not on my radar and will probably take some time to be resolved.

@ytmimi If that is of your interest, feel free to create another issue.

[ytmimi]

@c410-f3r thanks for following up. I'll go a head and create a new tracking issue for this soon!

[momvart]

Any updates about tracking this issue? It looks like it's got stuck in the preferred style decision, isn't it?