Align with latest decisions (early March 2023)

[ftomassetti]

"Inspired" by LionWeb-io/lionweb-typescript#48

• Change serialization format to use meta-pointers specification#93
• Add version property to M3 Metamodel specification#92 (waiting for decision)
• Requirements on metamodel keys specification#91 (PR Verifying the format of IDs and Keys #56)
• Rename M3 property id -> key specification#90
• Establish term meta-pointer specification#89
• Introducing the builtin interface INamed specification#86 (not yet confirmed)

[ftomassetti]

I should change the type of Metamodel.version from string to integer

[dslmeinte]

Didn't we agree for Metamodel.version to be a string?

[enikao]

It might even be both:

• integer in M3
• string in all places in serialization format

(just showing options here, not an expression of opinion/preference)