Add a function provides global access to the current page

[Tiscs]

Add a function(e.g. page) provides global access to the current page, likes the function site. It's useful when writing partial templates, such as need to check if a page is the .CurrentSection.

[bep]

See #9339

And I agree about the "usefulness". I have this more or less implemented in another branch, but I keep getting side tracked :-)

[github-actions]

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