Add support for airline warning and error colors

[axelitus]

I noticed there weren't colors defined for the errors and warnings of airline, so I've added those to the airline nord theme.

[axelitus]

Without warninng color:

With warning color:

[arcticicestudio]

Hi @axelitus 👋
Thanks for your contributions and patience 👍

I've been too busy with my "real" life and the development of the official Nord website to work on the massive amount of issues/PRs that were submitted each day for all of Nord's port projects.

Your changes looking good to me except that we should use nord1 instead of nord3 as foreground for warnings for better saturation and visual distinction to the background.
I've added the code change suggestions as comment to this PR.

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Release Note Assets

Before

After

[axelitus]

No worries, I know how too busy can feel overwhelming sometimes. I'll take a look at it later today or tomorrow.

[axelitus]

Totally agreed with your comments. It's better with more contrast.

I have added a commit which implements your suggestions.

[arcticicestudio]

I've rebased your branch on top of the develop branch and resolved the merge conflict and will merge it to deploy it with the next release version 🚀
Thanks again for the PR 👍