Support default value in decorators

[timotheeguerin]

extern dec encode(target: unknown, encoding: string, encodedAs?: string | bytes  = string);

Would be good to be able to do that.

To decide in design is do we want the property with default to be marked optional or not

extern dec encode(target: unknown, encoding: string, encodedAs?: string | bytes  = string);
// vs
extern dec encode(target: unknown, encoding: string, encodedAs: string | bytes  = string);

In typescript they have to be marked required

[markcowl]

need to decide on syntax where default values are allowed

[markcowl]

related to: #1955