feat: add outline-fade less variable

[Satloff]

🤔 This is a ...

• New feature
• Bug fix
• Site / document update
• Component style update
• TypeScript definition update
• Refactoring
• Code style optimization
• Test Case
• Branch merge
• Other (about what?)

💡 Background and solution

Some users may want to change the opacity of the face surrounding forms when active. Adding a variable called outline-fade to acomodate that.

📝 Changelog

Language	Changelog
🇺🇸 English	adds @outdline-fade variable to default.less
🇨🇳 Chinese

☑️ Self Check before Merge

• Doc is updated/provided or not needed
• Demo is updated/provided or not needed
• TypeScript definition is updated/provided or not needed
• Changelog is provided or not needed

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[Satloff]

@afc163 can you review this?

[zombieJ]

Hi @Satloff, master only accept bug fix since v3 is in maintaining. Could you help change this PR base from master to 4.0-prepare branch?

Thanks

[yoyo837]

master merged 4.0 a few days ago.

[ant-design-bot]