Add pattern completion for unapply contexts

[natsukagami]

Fixes #19972.

Add pattern completion for Unapply tree contexts.

A typical example would be

optionList match
    case List(S@@)

which should be prompted Some(value), due to List.unapplySeq expecting Option[T] patterns as arguments.

• Add unapply pattern completion test
• Add pattern completion based on the Unapply argument type