Markup warnings/lint

[HugoDF]

Additional things we can write our own "warning" for (see alpinejs/alpine#975)

• x-init on an element without x-data (v2)
• Alpine directives outside of x-data
• Alpine is loaded but there is no x-data on the page
• an alpine directive is being used on an element that doesn't have x-data and has no element with x-data in its ancestors
• x-if/x-for on a non-template element
• template with more than 1 child node

We need to have a think about how to highlight template's that have issues (they're visually hidden... maybe having warnings is enough).

Originally posted by @HugoDF in #126 (comment)

[markfirmware]

x-for requires a non-template child (whereas x-if does not require this)

There are only three valid templates (these, and only these, attributes may appear)

1. x-if
2. x-for
3. x-for :key

[HugoDF]

Maybe relevant discussion for a warning alpinejs/alpine#1038