feat: Add use-layers rule

[nzakas]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

Add a new use-layers rule

What changes did you make? (Give an overview)

• Added use-layers rule
• Added tests
• Added docs
• Ensured the rule is exported from the plugin

Note: This rule is not recommended because it will likely only be useful on a subset of CSS files in any given project.

Related Issues

fixes #18

Is there anything you'd like reviewers to focus on?

• Should @import require a layer by default? Right now it doesn't, but I feel like maybe it should. Yes!
• CSSTree is returning the wrong location for layer names. I've opened an issue for that: Bug: Incorrect location for @layer prelude csstree/csstree#309

[jeddy3]

Looks great!

Should @import require a layer by default? Right now it doesn't, but I feel like maybe it should.

I think so. I had the same thought before seeing this PR.

[nzakas]

@fasttime I can't quite figure out why the type tests are failing now. Can you take a look?

[nzakas]

@jeddy3 thanks, updated!

[nzakas]

I think I figured out the types issue: #28

[nzakas]

I fixed the bug in CSS Tree with layer positioning. Just waiting for a new release to update this PR.

[jeddy3]

I've added two suggestions for extra tests.

I think I'd find it more intuitive if the boolean options matched, i.e. both made the rule stricter or more permissive.

For example, either:

• allowUnnamedLayers: false
• allowUnLayeredImports: false

or:

• requireNamedLayers: true
• requireImportLayers: true

[nzakas]

With CSSTree updated, all of the locations are now correct. 🎉

[mdjermanovic]

LGTM, thanks!