chore: update postcss version to 8

[gu-stav]

This updates the postcss version to 8 and moves the dependency to peerDependencies. Tests are all passing, but there are a couple of deprecation warnings.

Fixes #324

[codecov]

Codecov Report

Merging #325 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #325   +/-   ##
=======================================
  Coverage   92.68%   92.68%           
=======================================
  Files          10       10           
  Lines         328      328           
  Branches      115      115           
=======================================
  Hits          304      304           
  Misses         23       23           
  Partials        1        1           

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[ai]

LGTM

[wardpeet]

If we move it to peerdependencies, it's a breaking change. You'll have to give us some time to see if we can bundle more breaking changes.

[ghost]

Hi, I'm curious what the status is? I'm waiting for an update to be released so I can use postcss 8 plugins with this rollup plugin...

[isaacl]

I didn't realize there was a PR and uploaded #335. Mine updates other deps that support postcss 8 and updates test snaps, I think it's probably better in the end.

[patagoniapy]

Any updates on when this can be merged? Not being able to use postcss-8 over two months down the road is killer.

[SASUKE40]

LGTM

[egoist]

🎉 This PR is included in version 4.0.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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