Chore: Rename .stylelintrc to .stylelintrc.json

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This is recommended by Stylelint themselves and mean the file is syntax highlighted in IDEs and GitHub Web UI and will be linted as JSON to help prevent any potential issues.

Preview: https://github.com/coliff/webhintio.github.io/blob/patch-2/.stylelintrc.json

webhintio / webhintio.github.io

Chore: Rename .stylelintrc to .stylelintrc.json #1166

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