remove `isNodeFromTemplate` hacks to expose elements without an ownerKey

[caridy]

Description

The method isNodeFromTemplate is confusing on itself, its semantics are really about whether a node is inside a shadow or not, maybe we should rename it to isNodeShadowed or something similar.

There is one use-case today that is forcing us to break those semantics for a very edge case:

The Problem

• when a node is inserted manually inside an element from template, and that element does not have the lwc:dom="manual" flag specified, the node is considered as not part of the shadowRoot when running in synthetic mode. This is considered a feature for flexipages inserting aura elements inside the flexipage, while those aura elements are accessible via a global document query.

This is plain wrong, and we should fix it, but first we need to investigate how to help those folks that are using this as a feature.

[caridy]

/cc @diervo @ravijayaramappa @adheip-sfdc

[adheip-sfdc]

I'll have to defer to @diervo on this since it was his recommendation. I can definitely help with any changes that need to be made in flexipages, as long as we can find an alternate solution that won't regress customers.