Add wrapMultilineConditionalAssignment rule to wrap if / switch expressions to new line after assignment operator

[calda]

When discussing adopting the conditionalAssignment rule within Airbnb, we decided that the resulting code is easier to read if the if / switch expression is wrapped to a new line:

// Before: not wrapped, harder to read at a glance since the if statement is shaped strangely
let planetLocation = if let star = planet.star {
  "The \(star.name) system"
} else {
  "Rogue planet"
}

// After: wrapped after the assignment operator, the if statement has the same shape 
// we're used to seeing elsewhere (where the `if` starts on a new line).
let planetLocation =
  if let star = planet.star {
    "The \(star.name) system"
  } else {
    "Rogue planet"
  }

This is also the formatting that is used in most of the examples from the Swift Evolution proposal that introduced this feature (SE-0380), so seems likely to be a pretty common formatting preference.

This PR implements a wrapMultilineConditionalAssignment that wraps conditional assignment expressions in this way.

This PR also updates the indent rule to support this formatting.

[calda]

@nicklockwood, all of the tests are passing now on this PR now that I found a solution for #1575

[codecov]

Codecov Report

Attention: 107 lines in your changes are missing coverage. Please review.

Comparison is base (e6928ea) 94.99% compared to head (727b791) 95.07%.
Report is 26 commits behind head on develop.

❗ Current head 727b791 differs from pull request most recent head 521698a. Consider uploading reports for the commit 521698a to get more accurate results

Files	Patch %	Lines
Sources/GrammaticalNumber.swift	41.97%	47 Missing ⚠️
Sources/Options.swift	85.35%	23 Missing ⚠️
Sources/GitHelpers.swift	82.85%	18 Missing ⚠️
Sources/Rules.swift	98.12%	9 Missing ⚠️
Sources/ParsingHelpers.swift	94.80%	4 Missing ⚠️
Sources/FormattingHelpers.swift	98.17%	3 Missing ⚠️
Sources/Inference.swift	93.93%	2 Missing ⚠️
Sources/ShellHelpers.swift	96.42%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1574      +/-   ##
===========================================
+ Coverage    94.99%   95.07%   +0.08%     
===========================================
  Files           20       20              
  Lines        21845    21969     +124     
===========================================
+ Hits         20751    20887     +136     
+ Misses        1094     1082      -12     

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[calda]

Attention: 1 lines in your changes are missing coverage.

Impressive! The code coverage bot highlighted a case I forgot to add a test for. Added a test for it in 727b791

[calda]

I ran both the conditionalAssignment and wrapMultilineConditionalAssignment rules on the Airbnb codebase and didn't notice any issues, so this is totally good to go / ready for review now 👍🏻