add parsing support for supports conditions

[romainmenke]

see : #529

• added parsing support for supports conditions
• rewrite the import statement parser

---

Fully implementing supports conditions will require a fairly large refactor and the codebase is already very complex.

I would like to first do a few refactors.
This is one of them.

I've added c8 to ensure that test coverage remains high throughout this process.

File	% Stmts	% Branch	% Funcs	% Lines	Uncovered Line #s
All files	100	99.65	100	100
postcss-import	100	100	100	100
index.js	100	100	100	100
postcss-import/lib	100	99.34	100	100
assign-layer-names.js	100	100	100	100
data-url.js	100	100	100	100
join-layer.js	100	100	100	100
join-media.js	100	100	100	100
load-content.js	100	100	100	100
parse-statements.js	100	100	100	100
process-content.js	100	93.75	100	100	26
resolve-id.js	100	100	100	100

---

Initially the parsing algorithm was a simple split:

• look for anything that could be a uri
• everything after is a media condition

With layer and supports something more flexible is needed.
I've changed the parsing algorithm to a loop that keeps track of what has been encountered and errors on any unexpected sequences.

---

Any supports conditions will give errors:

Supports conditions are not implemented at this time.

This is a behavior change.
Before this PR supports conditions would be treated as if they were media conditions, which is also incorrect.

Both behaviors are "broken" but after this change CSS authors will receive more meaningful feedback.

[RyanZim]

@romainmenke are you thinking we should release with this warning for @supports, or is this just a temporary measure until we add support after refactoring?

[romainmenke]

are you thinking we should release with this warning for @supports, or is this just a temporary measure until we add support after refactoring?

I am hoping it is temporary.
But I want to keep the code base healthy after each change and I would like to keep each change small-ish.

The state in this pull request is something that can be released if needed.

I plan to continue work towards full support for supports conditions, but this could take a while. It really complicates things that four different types of rules interact (import, media, layer, supports). With layer I was able to patch a few existing bits and make it work, but a larger refactor is needed for supports.

[RyanZim]

Let's split the addition of c8 into its own PR.

Code here LGTM.

[romainmenke]

Thank you for the review 🙇