Support for dynamic values

[rtsao]

there's edge cases where truly dynamic values are needed.

this is usually some high cardinality domain of input that also needs to maps to a range of css values with similar cardinality.

Examples:

• Color picker (many colors, many css color values)
• Range slider (0-100 float, many css position values)
• Label placeholder (arbitrary strings, arbitrary placeholder content)

Ideally, ESS should provide a way to accommodate these use cases via some mechanism of dynamic values, provided that:

• XSS is impossible
• Build-time RTL is unaffected
• Build-time vendor prefixing is unaffected

Essentially, ESS needs to allow the things that would be safe and performant with CSS-in-JS, but disallow non-robust or unsafe things, or things that would require bulky runtime logic