"add/paste/move" elements validation mechanism

[edenvidal]

• if body is empty ,selecting it and adding an element from “add menu” inserts the element inside but when not, it gives validation error. @atulbhatt-system32 this is related.

• currently, adding things inside forbidden things is going crazy (try placing a link inside a link). here's a defined way to do it bottom-up.

note for beginners: this functionality involved contextual data coming from "rfrncs"

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• add menu: always show options - display all possible html elements in the add menu, no matter the current context.
• check context: when adding/moving/pasting an element, see if it fits in the current context.
• look for parent: if it doesn't fit, check higher-level contexts and add the element(s) there if it's allowed. if one element out of a few is violating, then the fallback rules applies to them too.
• ignore if not allowed: if no valid context is found, terminate the action and pop a notification for it - "Anchor inside Anchor is not allowed".

example 1: within an empty 'body,' adding a 'div' should place it inside the 'body.'
example 2: inside a 'ul' element and selecting a 'li,' adding another 'li' should put it next to the previous one. adding a 'div' will place it outside the 'ul.'

paste mechanism:

• follow same rules: use the same context rules when pasting elements.
• keep tree order: paste elements in the order they appear in the tree, not the initial selection order.

multi-element addition:

• all-or-nothing: if any element in a multi-addition is not allowed, cancel the entire operation.
• case-by-case handling: if all elements are allowed, apply the usual context rules to each one.