fix: crash in no-important on non-standard important values

[Pixel998]

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AI acknowledgment

• I did not use AI to generate this PR.
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What did you do?

/* eslint css/no-important: "error" */
a { color: red !ie; }

What did you expect to happen?

No error. !ie is a non-standard hack, not an !important flag.

What actually happened?

TypeError: Cannot read properties of null (reading 'index')

What is the purpose of this pull request?

This PR fixes the crash above. important holds the raw identifier for hacks like !ie, which is truthy, so the rule went on to locate the flag in the source text, found no match, and dereferenced null.

What changes did you make? (Give an overview)

Narrowed the check to important === true, so only real !important flags are reported.

Depends on eslint/csstree#152, which makes !IMPORTANT parse as true.

Related Issues

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

[DMartens]

Changes make sense to me, thanks. Marking as approved waiting for the csstree PR to land.