refactor(v2): extract site title formatter to theme-common util

[Simek]

Motivation

Currently the same function, same formatter is used in various packages to format the siteTitle.

This PR extracts the formatter function to stand alone hook (useTitleFormatter) and moves it to theme-common package.

@slorber My only concern is that after refactor I had to add @docusaurus/theme-common also to docusaurus-theme-search-algolia dependencies. In theory this is the expected that theme-search can be depended on theme-common, but due to the way in theme-common dependencies have been declared, this will mean that Algolia plugin now will fetch all content plugins (https://github.com/facebook/docusaurus/blob/master/packages/docusaurus-theme-common/package.json#L20).

Not sure how big problem it is in the general scope of Docusuasaurs, since this package should not be used without Docusuaurs, but maybe there is a better way to define/setup this. Maybe theme-common should only specify peerDependencies?

Have you read the Contributing Guidelines on pull requests?

Yes.

Test Plan

Changes have been tested using Docusuaurs V2 website locally.

Related PRs

• feat(v2): add ability to set custom title delimiter in config #3460
• feat(V2): add custom wrapper class to SearchPage, fix title #3834

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[slorber]

Thanks, that does not seem like a big issue to me.

CF the updated theme doc: https://v2.docusaurus.io/docs/api/themes

"main themes" depends on docs,blog, pages, but algolia search is a "theme enhancer" that improves one main theme, so it's not a big deal if it depends on the same things