Allow strong typing of the schema and make it instance independent

Currently, the schema type definition is loosely typed and each part of the schema (etype/attribute/relation schema) contains several eids. I think that we should "standardize" the way the schema is represented in ts/js. This single representation should be supported by the following tools:

cwclientlibjs
cubicweb-tsgen
rqlcontroller schema's holder
cwelements
rqlbrowser
ra-cubicweb

I think that the Schema type definition and its associated utils should be located in cwclientlibjs.

Edited Feb 24, 2021 by Frank Bessou

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