Add a few CSS rules

[43081j]

As part of expanding the ruleset to now work over lit's css static (which we should be able to access pretty easily now), these rules would be handy:

• css-deprecated-apply-rule to detect usages of @apply
• css-deprecated-var-args to detect usages of var(--a, --b) rather than var(--a, var(--b))
• css-host-display to detect if an element has set its display to something
• css-content-to-slotted to detect uses of ::content where we should use slotted
• css-host-hidden to enforce :host([hidden]) to be defined

[stramel]

My only question is should we spend time implementing the v0 spec to v1 spec changes? I believe most of the deprecated rules are based on https://github.com/Polymer/tools/issues/1051

It may also be worth implementing these in a better language appropriate linting tool, such as stylelint.

[43081j]

yup you are right, all of these belong in a stylelint plugin rather than here.

let's leave this for now until we figure what to do about that in general.

[43081j]

Closing this as it will soon be tackled in a separate related plugin.