Allow validation of code in a subdirectory

[timothyjward]

Pull request summary

I wanted to set up the EditorConfig-Action in my repository but found a couple of blocking issues:

• The .editorconfig and code to check is in a subfolder within the repository
• The sparse checkout from the actions/checkout plugin ends up owned by a different user and so needs to be marked as safe

Both of these changes are enabled using properties passed to the action.

You can see the result (from my fork) being used at Eclipse sensiNact

Additional information

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