Fix incorrect cascade layer order when some resources can not be inlined

[romainmenke]

Fixes the original problem described in #567 (comment)

The root cause is that all nodes are added to either the imports or bundle array in

postcss-import/lib/parse-styles.js

Lines 36 to 37 in d43ca15

	const imports = []
	const bundle = []

While @layer statements (those without a rule body) can precede @import.
To fix this issue I've added a new statement type for layers and added the needed mechanics to apply conditions to these. To express conditional @layer statements before other @import statements, or @layer statements that end up in between other @import statements we use base64 encoded stylesheets:

@import 'data:text/css;base64,QGxheWVyIGxheWVyLWJldGE=' print; is equivalent to @media print { @layer layer-beta } but is valid before other @import statements

I am unsure if it fixes all other reported examples as I don't have reproductions for those. I suggest they test out the fix (if/when it lands) and file new issues if they still encounter problems.

I verified the fix by re-testing with https://github.com/romainmenke/css-import-tests
With the patch applied 4 more test cases pass, without regressions in other cases.

[RyanZim]

Will release next week

[RyanZim]

Actually, there's no good reason for me to be the bottleneck for npm releases; @romainmenke I should add you on npm; just to confirm, you're https://www.npmjs.com/~romainmenke there?

[romainmenke]

Yes, that's me :)

[RyanZim]

@romainmenke Invitation sent; feel free to push the version bump/CHANGELOG update directly to master.

[romainmenke]

Thank you @RyanZim, I've released a new version.
Please do let me know if you spot any mistakes 🙇