(template-compiler) Using multiple if:true|false directives on same tag should be a compile-time error [API versioning]

[nolanlawson]

See #3277.

<template if:true={foo} if:false={bar}></template>

This should be a compile-time error. However, this would have backwards-compatibility concerns.

[nolanlawson]

This also gives a pretty misleading warning message today:

[git2gus]

This issue has been linked to a new work item: W-17406533