Proposal: Merge GlSkeletonLoading functionality into GlSkeletonLoader and deprecate GlSkeletonLoading

GlSkeletonLoading and GlSkeletonLoader have very similar functionality that could be combined to reduce confusion and make GitLab UI more maintainable.

Proposal

Changes to GlSkeletonLoader

• Add DEFAULT_LINE_WIDTH_PERCENTAGES as a constant from https://gitlab.com/gitlab-org/gitlab-ui/-/blob/master/src/components/base/skeleton_loading/skeleton_loading.scss#L4
• Add lines prop that defaults to 3 and is used to render the default slot value.
• Add equal-width-lines prop that defaults to false. The default lines will be the same width if this is true.
• Add default value to slot. This default value would be a SVG rectangle that is rendered based on the lines prop, equal-width-lines prop and DEFAULT_LINE_WIDTH_PERCENTAGES constant.

Changes to GlSkeletonLoading

• Add deprecated message. "This component is deprecated, please use GlSkeletonLoader."