Vue: Support nullish-coalescing-operator and optional-chaining in template

[ppjjzz]

they are at stage 3 now. Can support them in template?

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[posva]

Check https://github.com/vuejs/vue/issues/6307

[oliverzy]

Right now optional chaining has been officially part of ECMAScript 2020 (Chrome 80), therefore I think maybe we should reconsider this feature request again.

[emdahlstrom]

This would clean up template logic alot when working with nested fields from a cms like contentful

[jacekkarczmarczyk]

https://github.com/vuejs/vue-next/commit/8449a9727c942b6049c9e577c7c15b43fdca2867