Fix else-block update in keyed each-block

[rproserpio]

This fixes #4536 and fixes #4549.
The value of the updated array was in scope only in the each block and not in the else block.
The bug was likely introduced in #4413.
I've chosen to remove the shadowing of the this.vars.each_block_value variable to keep the behavior of the unkeyed case i.e. the let binding in the fragment initialization is updated during the update.

Thanks to @bwbroersma for the help and test case.

[Conduitry]

I'm not positive this is the correct solution - or, at least, it doesn't seem to precisely undo the problem that was introduced in #4413. Comparing the output in 3.18 and 3.19, I see that even in 3.18, we were declaring a new variable for the each value here, not using the block/fragment-scoped one.

I'm not sure what other effects overwriting the higher-up variable would have, but what seems to me to be more in the spirit of what was happening before is to just move the const ${this.vars.each_block_value} = ${snippet}; outside the scope of the new if block.

@tanhauhau do you have any thoughts on this?

[tanhauhau]

@Conduitry found the root cause, created a PR #4558

[rproserpio]

Oh, nice. I'm closing this PR, then. Thank you

[rproserpio]

By the way, the let binding in the fragment scope keeps a reference of the array as initialized. I don't think it's needed or useful as it could be modified by a pattern like

list.push(el)
list = list

[tanhauhau]

thanks!