Allow negative theme lookup of space objects

[kenny-f]

Currently, if you set your space to be an object in theme instead of an array then using for example -xl does not work.

Example:

// theme.js
const theme = { space : { s: '8px', m: '16px' } } 

// some-component.js
<div
  sx={{
    margin: '-m'
  }}
/>

The -m does not get replaced with -16px.

What we currently have to do is:

// some-component.js
<div
  sx={{
    margin: (theme) => `-${theme.space.m}`
  }}
/>

which is not as nice.

This pr will see if the value starts with a - and return the theme lookup value and add a -.

[jxnblk]

Thanks! This seems like a helpful addition to the API :)