Refactor loader to loading, match styles to spec

[dancormier]

@ttaylor-stack this PR refactors your PR to match the specifications in SPARK-117. Most notably:

• The component is named Loading instead of Loader
• The animation is lifted from the icon (if you inspect it in Stacks-Icons, you can see inline styles for the icon)
• The base component name was .s-loader--block. The --* suffixes are reserved for component child elements.
• Sizing is fixed
• CurrentColor is used for background color so it matches context

What's still needed to complete ticket

• Button docs need to be accurately updated
• Visual regression images need to be updated
• Follow up ticket is created for the logo-based loading animation
  • Note: I anticipate this will be a different component in Stacks Classic if included there at all.

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