feat(core)!: remove headers field from page data

[Mister-Hope]

This reduces page data size by removing headers info from page data, downstream themes can infer headers via dom structure with @vuepress/helper and they can manually write it to page data if necessary.

[coveralls]

Pull Request Test Coverage Report for Build 11266238703

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 71.722%

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Totals
Change from base Build 11266064405:	0.0%
Covered Lines:	648
Relevant Lines:	906

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💛 - Coveralls

[meteorlxy]

Then the markdown.headers option should default to false?

[meteorlxy]

Could headers be server-side rendered after the change?

[Mister-Hope]

I think the header information might still be required on the page object, but we should no longer pass it to client data by default.

[meteorlxy]

I see. What about SSR? Seems that helper pkg is using DOM api to get headers.