Implement safe navigation/null-coalescing operator

[masak]

Code like this:

my bossName;
if employee != None {
    if employee.boss != None {
        bossName = employee.boss.name;
    }
    else {
        bossName = None;
    }
}
else {
    bossName = None;
}

Could profitably be rewritten by using a ?. safe navigation operator:

my bossName = employee?.boss?.name;

[masak]

I decided not to refer to this as "the Elvis operator", since Wikipedia seems to be of the opinion that that's the name of another operator, usually spelled ?: but with the same semantics as 007 (and Perl 6) infix:<||>.

[masak]

The TC39 proposal makes a good point I hadn't considered: the operator is short-cricuiting such that any side effects happening on the rhs of a ?. with a none lhs won't run. We want to do it that way too, I'm sure.