Using a read-only var name as a for loop var should be an error

[krader1961]

While working on #4200 I noticed that if you run this:

false
for status in 3 2 1
    echo $status
end
echo $status

You see this:

1
0
0
0

Rather than this (or an error):

3
2
1
0

Contrast with this:

$ set status 1
set: Tried to change the read-only variable 'status'

[krader1961]

I'm retargeting this for fish 3.0 because it is a backward incompatible change that can only be done in a major release. Now obviously anyone currently doing this type of thing is seeing unexpected, incorrect, behavior. Nonetheless, we should only fix it in a major release so let's make sure we do so for 3.0.