Add systemd icon

[ultram4rine]

This adds icon for systemd units and for systemd-conf language from this extension. File extensions taken from Red Hat docs. Icon taken from systemd brand site.

Colors changed from #30D475 to #00E676 and from #201A26 to #212121 using converter.

Closes #1163

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[ultram4rine]

In e917585 I try to follow up #1310 and make icon pixel perfect. Result is minimal, but I think it's a little better now, at least the brackets. On the screenshots below service is a new one, target is old.

Preview

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Light:

[github-actions]

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[okineadev]

When will we accept PR?

[github-actions]

Preview

Thank you for creating a pull request. This preview shows you how your icons will look on the different themes:

Check how your icons fit in a 16x16 grid with our Pixel Perfect Checker by following this link.

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[PKief]

When will we accept PR?

The icon looks very tiny and is not following the grid:

Maybe we can improve this a little bit to make the icon visually better.

[PKief]

and color checks are failing too:

We should replace the white color with a light grey

[PKief]

Additionally, I've fixed the light mode for language ids. There was a problem before (probably due to some refactoring in the past): f532fb2

[PKief]

Icon is aligned to the 16x16 grid now:
https://pixp.lucode.ar/material-extensions/vscode-material-icon-theme/pull/1472

[github-actions]

Merge Successful

Thanks for your contribution! 🎉

The changes will be part of the upcoming update on the Marketplace.