fix binding group with each

[tanhauhau]

Fixes #3243

This builds on top of #4861, therefore contains the commit of the PR

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Tests

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[Conduitry]

@tanhauhau Can you take a look at the merge conflicts here as well please? I don't think these are all caused by bindingGroup/binding_group stuff either.

[tanhauhau]

@Conduitry conflicts resolved.

[Conduitry]

Hm. Can you explain more precisely what this is intended to fix? I'm trying out this branch against the various examples given in REPLs in #3243 and I'm not sure what I'm supposed to be seeing. I see different behavior than I am in the version linked to in the REPLs, but in several cases nothing seems to be happening now when clicking on different checkboxes.

[tanhauhau]

so previously sharing the same binding_group is based on the variable name, eg:

<input bind:group={a} value="a">
<input bind:group={a} value="b">
<input bind:group={b} value="c">
<input bind:group={b} value="d">

{a} {b}

"a", "b" share the same binding group a, "c", "d" share the same binding group b.

however in a {#each}, this may break

{#each array as item}
    <input bind:group={item} value="a">
    <input bind:group={item} value="b">
    {item}
{/each}

svelte repl

although the bind:group uses the same variable name, item, they should be bound to a different binding group.

the current behavior is that they all share the same binding group.
So you would notice after selecting all the checkbox in the first each item, selecting any checkbox in the 2nd each item will select all the checkboxes within them.

The fix is to figure out if the variable name is under a each context, and treat each {#each} item bind:group={item} binds to a different binding group