Client rework.

[mshima]

At .Net blueprint the client is entire inside a sub-folder, except some files like README that is written to the application root.
The focus is to convert files relative to root and improve reuse of jhipster generated resources.

• add relativeDir/relative to base
• make paths relative to clientRootDir when required at angular, react and vue.
• convert some files to .jhi
• move some configs to new structure

Related to jhipster/jhipster-dotnetcore#1358 and #23449.

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• All continuous integration tests are green
• Tests are added where necessary
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• Documentation is added/updated where necessary
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