Create unit helpers px(), em(), rem(), percent(), viewHeight(), viewWidth()

[notoriousb1t]

Acceptance criteria:

• Should go in src/units.ts
• Should have at least one test in src/tests/units.ts
• Should export in src/index.ts
• Signatures should be:

px(val: number): CSSLength em(val: number): CSSLength rem(val: number): CSSLength viewHeight(val: number): CSSLength viewWidth(val: number): CSSLength percent(val: number): CSSPercentage

[notoriousb1t]

Added in https://github.com/typestyle/csx/commit/a0ac5b433a28bbd6f9362baf779ce9e82ed5c04f

[basarat]

Sweet 🍬 :rose: