Create viewer : Histogram

A first viewer should be implemented.
Here are its prereqs :

• must inherit from HTMLElement and manage its shadow DOM
• must have a slot to give it HERA-formatted data (see below)
• should have at least two other configuration slot (size, color, line stroke style idk)
• must be defined in its own JS module, so third party apps can import it with a one liner this implies that the viewer/module manages its own dependencies (ie. D3)

Functionnaly-speaking, this first viewer can be anything, but I advice it to be something simple, like an histogram (one integer metadata for each item) or a basic graph (2 numeric data for each item).