Add new file_content_type for PreviewProvider subclasses

[Jeehut]

Fixes #2860.

[SwiftLintBot]

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🚫	Please rebase to get rid of the merge commits in this PR

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📖	This PR fixed a violation in Alamofire: /watchOS Example/watchOS Example WatchKit Extension/ContentView.swift:28:1: warning: File Types Order Violation: A 'main_type' should not be placed amongst the file type(s) 'supporting_type'. (file_types_order)

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[PSchmiedmayer]

@Jeehut I think this is a great addition to the file_content_type rule, thanks!
@PaulTaykalo @ZevEisenberg Any update to when it would be possible to merge this into master? Any way I could help there?

[M0rtyMerr]

@Jeehut @PaulTaykalo @ZevEisenberg this feature is mentioned in 0.39.0 release notes in CHANGELOG, however, it's not available in pod, which is very confusing and looks like an accidental mistake. Is there anything blocking that from release? It would be awesome to get this feature :)
Sincerely