Add support for destructuring assignment in t-as

[sdegueldre]

Example of things that would be nice to support:

<t t-foreach="[[1, 'test', 'otherValue'], [2, 'test2', 'somethingElse']]" t-as="[key, value, otherValue]" t-key="key">
  <t t-esc="value"/> is <t t-esc="otherValue"/>
</t>

[sdegueldre]

Question: what happens with $as_value, $as_index, etc

[ged-odoo]

maybe we could stop supporting them? and if necessary, add t-as-index to give an explicit var name. (obviously, for owl 3 then)

[sdegueldre]

(backwards compatible alternative: add a different t-for/t-for-of that doesn't need to support these)

[sdegueldre]

backwards compatible proposal: #1491