New sections in the CSS reference article for the shadow and easing editors

[captainbrosset]

The shadow and easing editors are not documented yet. This PR adds two consecutive H2 sections in the CSS reference article to fix this.
Fixes #2755.

Article previews:

• GitHub: https://github.com/MicrosoftDocs/edge-developer/blob/user/pabrosse/easing-editor/microsoft-edge/devtools-guide-chromium/css/reference.md#change-box-and-text-shadows-with-the-shadow-editor
• Preview server: https://review.learn.microsoft.com/en-us/microsoft-edge/devtools-guide-chromium/css/reference?branch=pr-en-us-2798#change-box-and-text-shadows-with-the-shadow-editor

AB#46102135

[mikehoffms]

main Writer/Editor review pass

[mikehoffms]

Final pass of Writer/Editor review

[learn-build-service-prod]

Learn Build status updates of commit 69e0521:

✅ Validation status: passed

File	Status	Preview URL	Details
microsoft-edge/devtools-guide-chromium/css/reference.md	✅Succeeded	View
microsoft-edge/devtools-guide-chromium/css/reference-images/easing-editor.png	✅Succeeded	View
microsoft-edge/devtools-guide-chromium/css/reference-images/easing-editor-changes.png	✅Succeeded	View
microsoft-edge/devtools-guide-chromium/css/reference-images/easing-editor-icon.png	✅Succeeded	View
microsoft-edge/devtools-guide-chromium/css/reference-images/shadow-editor.png	✅Succeeded	View
microsoft-edge/devtools-guide-chromium/css/reference-images/shadow-editor-changes.png	✅Succeeded	View
microsoft-edge/devtools-guide-chromium/css/reference-images/shadow-editor-icon.png	✅Succeeded	View

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