feat(client): allow html title customization (close #338)

[meteorlxy]

Close #338

This PR allows users to customize the content of <title> tag in this way:

// clientAppEnhance.ts
import { defineClientAppEnhance, resolvePageHeadTitle } from '@vuepress/client'

export default defineClientAppEnhance(() => {
  resolvePageHeadTitle.value = (page, site) => `${page.title} | ${site.title}`
})

[meteorlxy]

cc @shigma @Mister-Hope @jiazengp

[shigma]

To me this feature is more like a configuration. It will be quite annoying if one has to write lots of different composition functions for every similar configurations.

I suggest that we only provide a simple client API that exposes all configurations.

// clientAppEnhance.ts
import { defineClientAppEnhance, useClientAppConfig } from '@vuepress/client'

export default defineClientAppEnhance(() => {
  const config = useClientAppConfig() // returns reactive object
  config.resolvePageHeadTitle = (page, site) => `${page.title} | ${site.title}`
})

or we can even provide a syntactic sugar for the code above:

// clientAppEnhance.ts
import { defineClientAppEnhance } from '@vuepress/client'

export default defineClientAppEnhance({
  resolvePageHeadTitle: (page, site) => `${page.title} | ${site.title}`,
})

In that case we can easily extend our client app configuration in the future.

[meteorlxy]

@shigma Sounds good to me.

What other config can be put into it (or in the future) ?

[Mister-Hope]

My opinion:

I am not coming up any idea what config need to added on the client side, but I think maybe we could extend more config in user config or core.

Some of them already appears in issue or dicussion, and are reasonable.

E.g.:

• Desides whether accessing other pages data sync or async. Small docs or blog sites could benefit from visiting a sync pages array. And large sites can increase speed when asynchronously load each page data.

• add darkmode comparability and its status to core. These will make the srr render result default to dark possible, and help hold the same config and behaviour across plugins and themes.

[drewcook]

I'd like to add that there may be complexity when using extended page data. Please see my issue here - #487

How will this PR address, if at all, using extended page data into defineClientAppEnhance?