Syntax: Default value for fields

Derived from discussion about parent issue #216 (closed) within LC-Types:

People from the domain model asked for a less verbose option to set default values.

The M-SDL proposal uses an explicit keep statement for that:

my_int: int with:
    keep(default: my_int == 1)

For less verbose and simpler readability the original python syntax was proposed, which is semantically the same:

my_int: int = 1