move to TypeScript

[LitoMore]

Changes

• Rewrite with TypeScript
• Separate get and to functions to support tree-shaking
• Expose some useful utilities
• Use c8 & ava for testing
• Use named capturing group for getting matched values
• Change all null return values to undefined (FYI Intent to stop using `null` in my JS code sindresorhus/meta#7)

Some lines are theoretically unreachable (unless the regex pattern has a bug). For TypeScript to determine the type, I wrote a polite condition such as if (!r || !g || !b) return undefined and ignored its coverage.

Notable changes

• Now all get and to function parameters will be clamped
• Fix some issues with negative numbers

   colorString.to.hex(255, 10, 35, -1)
   //=> FF0A23-FF
   
   toHex(255, 10, 35, -1)
   //=> FF0A2300
  
   colorString.to.rgb(-255, -10, -35)
   //=> rgb(-255, -10, -35)
  
   toRgb(-255, -10, -35)
   //=> rgb(0, 0, 0)