feat(core): support injecting meta into router

[Mister-Hope]

No description provided.

[Mister-Hope]

Here I deep merge meta key to avoid the frontter edits being coverd.

But, I think there are issues with this design. I am explaining why there are problems in #519 .

Though this simple change avoid #519, but users are not able to delete any meta keys.

[Mister-Hope]

I think it's ready for the feature, improvements can make later together with "frontmatter" when having later discussions

[meteorlxy]

Sorry for delay. (Really busy and kind of tired these days 😅 ) I'll pick up this PR soon.

[meteorlxy]

As meta is already available in the route.meta, is it necessary to put meta in to page data?

I think we can make meta a property of the Page object, and don't need to put it into page data.

What's your opinion? @Mister-Hope

[Mister-Hope]

Sounds ok to me.

[meteorlxy]

If we remove it from page data, we could not use extendsPageData to modify it. I have two proposals:

1. Let users to modify it manually in onInitialized hook:

onInitialized: (app) => app.pages.forEach((page) => {
  page.data.foo = 'foo'
  page.routeMeta.title = page.title
  // ...
})

In fact, the extendsPageData hook could also be removed as we have added page.data property, and users can modify it in this way, too. 😅

2. Rename extendsPageData to extendsPage:

extendsPage: (page) => {
  page.data.foo = 'foo'
  page.routeMeta.title = page.title
  // ...
}

The only difference is that plugins / themes don't need to traverse the app.pages array by themselves.

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@Mister-Hope

[meteorlxy]

I've replaced extendsPageData with extendsPage in 827a873.

Then this PR could be implemented in a different way.

[meteorlxy]

Implemented in 93cdb53