[Static optimization] Allow key directive

[nolanlawson]

Right now, for the following template:

<template>
    <table>
        <tbody>
            <template for:each={rows} for:item="row">
                <tr key={row.id}>
                    <td>hello</td>
                    <td>world</td>
                </tr>
            </template>
        </tbody>
    </table>
</template>

...we are not able to hoist the entire <tr> into a single static block, because the key directive disallows this.

We can potentially support the key directive – but only at the top level of the block. (Inside of the block, it's possible to use a key, but it's ignored with a warning: #3375)

[git2gus]

This issue has been linked to a new work item: W-15307004