[Blocked] Update code coverage to include JS expressions in <template>s of Vue components

[trusktr]

EDIT: Need to update existing code coverage documentation with a blurb about accuracy and a link to this issue

I was unable to get instrumentation into the JS expressions inside <template>s of Vue components. I asked a couple questions for ideas here:

• unable to instrument Vue template code when using vue-loader istanbuljs/babel-plugin-istanbul#273
• Any idea how to get this to use babel-plugin-istanbul for coverage in templates? JuniorTour/vue-template-babel-compiler#38

Originally posted by @trusktr in #4649 (comment)

[unlikelyzero]

Note: this is functionally blocked based on the two other issues linked above. We need to attempt to run through vuejs/vue-cli#1363 (comment)
but it may lead to a dead-end