Add shorthand expansion and compression to the lexer

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What this task asks for

Add lexer methods to expand CSS shorthands into longhands and compress longhands back into shorthand values. Add two methods to the lexer: `expandShorthand(propertyName, value)` expands a CSS shorthand into an object mapping each longhand name to its value string. `compressShorthand(propertyName, longhands)` compresses an object of longhand name-value pairs back into a shorthand value string. When a component is omitted from the value, the corresponding longhand receives its CSS initial value. Must support at minimum: margin, padding, border, border-top, border-right, border-bottom, border-left, background, font, outline, overflow, flex, flex-flow, gap, text-decoration, list-style, inset, and border-radius. Must work with custom syntax created via fork(). Expanding a shorthand and then compressing the result should produce an equivalent shorthand value.