Add polyfill for inset-start and inset-end

Editor assessment

The subject states the addition precisely: polyfills were added for the inset-start and inset-end style props in the styled system. The styled-system scope is a proper package noun, the fix type matches a gap correction, and both logical properties are named, so the reader knows exactly what started working. There is no body, acceptable for a small polyfill addition whose headline enumerates the surface. No footers appear. The lowercase imperative phrasing is clean. Compact, specific, and self-sufficient. Naming both logical properties makes the fix verifiable against the style-prop list.