Evaluate theme functions in plugins

[adamwathan]

This PR fixes a bug where CSS theme() functions were not evaluated when present in rules added by plugins, using either @plugin or registering a plugin in a JS config file.

For example, prior to this PR the theme() functions in this plugin would make it into the final CSS without being evaluated:

// ./my-plugin.js
export default plugin(({ addBase }) => {
  addBase({
    '.my-rule': {
      background: 'theme(colors.primary)',
      color: 'theme(colors.secondary)',
    },
  })
})

[thecrypticace]

The theme() calls from utilities are not processed by this call to substituteFunctions. The call in build() handles that because the generated nodes are the ones that have theme().

This is purely required for the work where we're taking the config stuff and backfilling some values into the theme (which ultimately makes it into the "static" AST).

So maybe the comment could use some clarification.

[thecrypticace]

Though theme calls inside @utility would be processed here but that happens to be more of an optimization.

[philipp-spiess]

Yeah, this is only for detecting wether addBase pushed stuff into the input CSS AST, right? The rest will exist only in the designSystem.

[thecrypticace]

see comment above but this is where theme() calls output by utilities (doing like addUtilities or matchUtilities) are processed.

[philipp-spiess]

We should probably also add an addUtility to test values inside utilities (since these won't be added to the initial CSS AST so it will be replaced at a different layer.

Suggested change

	addBase({
	'.my-base-rule': {
	color: 'theme(colors.red)',
	'outline-color': 'theme(colors.orange / 15%)',
	'background-color': 'theme(--color-blue)',
	'border-color': 'theme(--color-pink / 10%)',
	},
	})
	addBase({
	'.my-base-rule': {
	color: 'theme(colors.red)',
	'outline-color': 'theme(colors.orange / 15%)',
	'background-color': 'theme(--color-blue)',
	'border-color': 'theme(--color-pink / 10%)',
	},
	})
	addUtilities({
	'.my-utility': {
	color: 'theme(colors.red)',
	},
	})

[philipp-spiess]

Yeah, this is only for detecting wether addBase pushed stuff into the input CSS AST, right? The rest will exist only in the designSystem.