Extend parser with '?//' destructuring alternative operator

[sshine]

There was an operator I'd overlooked because I thought that it was a composition of two other operators. But it gets treated like its own operator and has its own precedence, as documented here:

• JBOL operator precedence table
• jq manual: Destructuring Alternative Operator: ?//

The task is:

• Extend Expr with AlternativeDestructure constructor.
• Extend operator table in Parser with this operator.

One challenge lies in the character overlap between the ? postfix operator and ?//. It may be necessary to change the way the ? postfix operator is parsed in the same way as #28 addresses this for existing operators with a similar overlap (e.g. / and //).