feat(calendar): add content slot to render schedule

[Juzisuan965]

Types of changes

• New feature
• Bug fix
• Enhancement
• Component style change
• Typescript definition change
• Documentation change
• Coding style change
• Refactoring
• Test cases
• Continuous integration
• Breaking change
• Others

Background and context

Calendar组件如果通过default 插槽来实现日程（比如国定假日、节气、备注）的展示，会失去默认的日期点击事件。
#3464

Solution

新增content插槽，实现日程展示，保留默认的日期展示与点击事件。

How is the change tested?

人工测试来保证新增的slot正常工作，不影响其他内容。

Changelog

Component	Changelog(CN)	Changelog(EN)	Related issues
Calendar	新增 content 插槽	Added content slot	Close #3464

Checklist:

• Test suite passes (npm run test)
• Provide changelog for relevant changes (e.g. bug fixes and new features) if applicable.
• Changes are submitted to the appropriate branch (e.g. features should be submitted to feature branch and others
  should be submitted to main branch)

Other information

这个PR能够让Calendar提供类似日程的功能，而保留默认的日期展示、事件点击，便于开发者使用

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[Juzisuan965]

@oljc review下不？