feat: allow to load an XML-stored model produced by mxGraph - #300

Implementation relies on both the declaration of aliases and the introduction of dedicated codecs.
Codec.getCodecByName now takes aliases into account, so that importing is fully functional.
The code of some CodecRegistry and Codec methods has been simplified (check for nullity of method parameters).

The cell style is converted thanks to a specific utility function that can serve as an example for those wishing to migrate their applications from mxGraph to maxGraph.

test

• The Codec example has been restored from the stashed directory. It is now available as a story written in TypeScript.
• The ModelChecker utility class was previously used only to validate the imported model after import from the maxGraph model. It is now shared and also used in mxGraph import tests. It has also been enhanced to check the total number of cells in the model.

Closes #293

### Tasks
- [x] import vertices and edges
- [x] import cells geometry
- [x] restore the stashed Codecs story and migrate it to TypeScript
- [x] correctly import the edge geometry when it includes an array of points
- [x] import the style. Require conversion from string to object
- [x] remove extra logs
- [x] update PR description

Screenshots of the Codec story

For comparison with mxGraph: https://jgraph.github.io/mxgraph/javascript/examples/codec.html