Add unavailable_condition rule

[marcelofabri]

Fixes #3897

[SwiftLintBot]

	4 Warnings
⚠️	This PR introduced a violation in Aerial: /Aerial/Source/Views/AerialView.swift:182:12: warning: Unavailable Condition Violation: Use #unavailable instead of #available with an empty body. (unavailable_condition)
⚠️	This PR introduced a violation in Aerial: /Aerial/Source/Views/PrefPanel/InfoWeatherView.swift:41:12: warning: Unavailable Condition Violation: Use #unavailable instead of #available with an empty body. (unavailable_condition)
⚠️	This PR introduced a violation in Aerial: /Resources/MainUI/First time setup/VideoFormatViewController.swift:30:12: warning: Unavailable Condition Violation: Use #unavailable instead of #available with an empty body. (unavailable_condition)
⚠️	This PR introduced a violation in Realm: /examples/ios/swift/ListSwiftUI/Views/ContentView.swift:271:20: warning: Unavailable Condition Violation: Use #unavailable instead of #available with an empty body. (unavailable_condition)

	12 Messages
📖	Linting Aerial with this PR took 0.99s vs 0.98s on master (1% slower)
📖	Linting Alamofire with this PR took 1.12s vs 1.12s on master (0% slower)
📖	Linting Firefox with this PR took 4.24s vs 4.2s on master (0% slower)
📖	Linting Kickstarter with this PR took 7.51s vs 7.46s on master (0% slower)
📖	Linting Moya with this PR took 4.38s vs 4.39s on master (0% faster)
📖	Linting Nimble with this PR took 0.4s vs 0.4s on master (0% slower)
📖	Linting Quick with this PR took 0.18s vs 0.18s on master (0% slower)
📖	Linting Realm with this PR took 10.22s vs 9.99s on master (2% slower)
📖	Linting SourceKitten with this PR took 0.34s vs 0.33s on master (3% slower)
📖	Linting Sourcery with this PR took 2.04s vs 2.02s on master (0% slower)
📖	Linting Swift with this PR took 3.25s vs 3.22s on master (0% slower)
📖	Linting WordPress with this PR took 7.59s vs 7.53s on master (0% slower)

Generated by 🚫 Danger

[codecov-commenter]

Codecov Report

Merging #3953 (f800663) into master (291b500) will decrease coverage by 0.04%.
The diff coverage is 19.04%.

@@            Coverage Diff             @@
##           master    #3953      +/-   ##
==========================================
- Coverage   92.61%   92.57%   -0.05%     
==========================================
  Files         441      442       +1     
  Lines       22239    22260      +21     
==========================================
+ Hits        20597    20607      +10     
- Misses       1642     1653      +11     

Impacted Files	Coverage Δ
...ork/Rules/Idiomatic/UnavailableConditionRule.swift	10.52% <10.52%> (ø)
...tFrameworkTests/AutomaticRuleTests.generated.swift	100.00% <100.00%> (ø)
Tests/SwiftLintFrameworkTests/TestHelpers.swift	79.65% <0.00%> (+0.57%)	⬆️
Source/SwiftLintFramework/Models/Linter.swift	84.44% <0.00%> (+2.22%)	⬆️

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[SimplyDanny]

LGTM!