Create formatter for GitHub Actions

[Iron-Ham]

This pull request adds a formatter for use with GitHub Actions.

The output of this formatter is meant to work as an actions annotation, as per the Actions Docs.

For example, if I had a file within my Utilities module that had an unused function, it would be output this way:

::warning file=Modules/Utilities/Sources/Utility/Sequence+Contains.swift,line=13,col=10,title=unused::Function 'containsNone(_:)' is unused

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Usage

$ periphery scan --relative-results --format actions

[!NOTE] Failure to specify --relative-results will result in a usage error being thrown instructing the user to use --relative-results. GitHub Actions specifically wants relative paths.

When used in CI, we get results directly on pull requests.

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New class and methods:

• Sources/PeripheryKit/Formatters/ActionsFormatter.swift: Introduced a new ActionsFormatter class with a format method to format scan results. This class requires the relativeResults configuration to be set, otherwise, it throws a PeripheryError.usageError.

Enum extension:

• Sources/Shared/OutputFormat.swift: Extended the OutputFormat enum to include a new case actions.
• Sources/PeripheryKit/Formatters/OutputFormatter.swift: Updated the OutputFormat extension to return ActionsFormatter.self when the actions case is selected.

[ileitch]

Many thanks for this! I renamed the format to github-actions as actions by itself may be ambiguous.