Theme package: Add element size design tokens

[jameskoster]

What

Adds a size category to the dimension token family with eight design tokens for element sizing:

Token	Value	Description
--wpds-dimension-size-5xs	4px	Notification indicators
--wpds-dimension-size-4xs	8px	Visual size for small interactive elements like resize handles
--wpds-dimension-size-3xs	12px	Small markers
--wpds-dimension-size-2xs	16px	For small controls like checkboxes and radios, or small decorative icons
--wpds-dimension-size-xs	20px	For medium sized icons
--wpds-dimension-size-sm	24px	For icons and small buttons
--wpds-dimension-size-md	32px	For medium-sized buttons and inputs
--wpds-dimension-size-lg	40px	Default size for buttons and inputs

Why

Components like buttons, checkboxes, icons, tabs, and other interactive controls currently use hardcoded pixel values for sizing. A shared set of size tokens provides a consistent, predictable scale that can be referenced across components, reducing ad hoc values and making it easier to maintain visual consistency.

How

• Added a size group to tokens/dimension.json with 5xs through lg entries, each referencing an existing space primitive (space.10 through space.80) so sizes align with the base spacing scale (4px–40px), including a 20px step at xs via space.50.
• Added an ElementSize TypeScript type in terrazzo.config.ts so the scale is available as a union type ('5xs' | '4xs' | '3xs' | '2xs' | 'xs' | 'sm' | 'md' | 'lg').
• Rebuilt prebuilt outputs (CSS custom properties, JS token lists, fallback values, TypeScript types, and docs).