Expose flattenColorPalette

[philipp-spiess]

Resolves #15315

It looks like we implemented this in Core but forgot to expose it from the distributed package (the references are only used for testing plugins internally right now). This exposes flattenColorPalette under the old import path of tailwindcss/lib/util/flattenColorPalette.

Test Plan

Added the following plugin to the Vite example and ensured it works as expected:

import flattenColorPalette from 'tailwindcss/lib/util/flattenColorPalette'
import plugin from 'tailwindcss/plugin'

export default plugin(({ matchUtilities, theme }) => {
  matchUtilities(
    {
      'hover-bg': (value) => {
        return {
          '&:hover': {
            backgroundColor: value,
          },
        }
      },
    },
    { values: flattenColorPalette(theme('colors')) },
  )
})

[thecrypticace]

ngl kinda sad we have to export this one but makes sense.

[thecrypticace]

Left some notes about stuff but generally looks good

[RobinMalfait]

Was a bit confused by this one, but makes sense!

[philipp-spiess]

lol yeah my bad when I merged the other one 😕