Make negative values a first-class feature, rather than theme-driven

[adamwathan]

This PR makes the negative prefix in Tailwind (like -mt-4) a true feature of Tailwind, rather than something that is generated because you have negative values in your config file.

Using a negative prefix in a key in your config file will still work the way it always has and will take precedence over this new "automatic" negative value support, but this PR does remove all the explicit negative values from the default config.

With this change, you can now use negative versions of anything "negatable" automatically, whether or not it's defined in your config, and even including arbitrary values:

Class	CSS
-mt-2	margin-top: -0.5rem
-mt-[5px]	margin-top: -5px
-mt-[-10px]	margin-top: 10px
-mt-[var(--whatever)]	margin-top: calc(var(--whatever) * -1)
-mt-0	margin-top: 0
-mt-auto	Ignored

This adds a lot of new power to Tailwind, while also letting us really simplify the default config. It also fixes a common hang up people run into where they expect -mt-[500px] to work, and I've always had to tell them to write mt-[-500px] instead.

There is a tiny breaking change here, which is that the negative helper function passed in to theme callbacks no longer returns the untouched value for values that cannot be made negative, for example:

negative({
  1: '1px',
  2: '2px',
  auto: 'auto'
})

// Old output
{
  '-1': '-1px',
  '-2': '-2px',
  '-auto': 'auto'
}

// New output
{
  '-1': '-1px',
  '-2': '-2px',
}

The getClassList test is currently failing, because completions were relying on the config information to generate the negative values. We will need to fix this before merging, so going to leave this open until then.