Specify icon creation/testing/maintenance guidelines

[oleq]

Until recently most of the icons were created by @oleq so there was little need for developing any guidelines. But if we want more developers to contribute, we need to make sure:

• Icons look well on Hi and Lo-DPI screens.
• Icons are stroke, not fill–oriented (except for maybe font background color), but generally speaking, we use icons that are "empty",
• We use 1,5px for most (but not all) strokes
  • What are the exceptions?
• We (more or less) always use rounded corners,
  • How much are they rounded?
• Icons have proper viewBox (viewBox="0 0 20 20")
• Icons must be monotone but "colorizable",
• Icons are cleaned up (yarn clean-up-svg-icons packages/**/theme/icons),
  • When? By whom?
• That developers know we have a manual test to compare all icons across the project (http://localhost:8125/ckeditor5-theme-lark/tests/manual/iconset.html) so they are similar in design and shape
  • Developers must update the test as they develop new icons for the project.
• ...and plenty more.

[oleq]

An excellent example of how to this right: https://material.io/design/iconography/system-icons.html#design-principles.

[CKEditorBot]

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[CKEditorBot]

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