Fix insertNodes insert position at start of inline ElementNode

[zurfyx]

insertNodes does too much...

insertNodes is a function that's able to insert N amount of nodes and fixed the hierarchy accordingly.

In #1996 we determined that insertNodes [x, y, z] where y is an ElementNode should never be merged into z. That makes sense, from a user PoV these are 3 separate nodes (an array) and you'd want them one next to the other.

However, when you insertNodes from a paste PoV, if your target is non-empty ElementNode you'd want to insertBefore the Element. For example, insertNodes[$createLineBreakNode()] with the aforementioned target should prepend it on the list of nodes.

Where do we draw the line? We don't, we reduce insertNodes responsibilities. We can assume that the first node's target can be treated as merge with the current hierarchy, whereas targets for subsequent node's should always be appended.

Screen.Recording.2023-10-10.at.3.54.44.PM.mov

Closes #4172

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size-limit report 📦

Path	Size	Loading time (3g)	Running time (snapdragon)	Total time
packages/lexical/dist/Lexical.js	28.28 KB (+0.03% 🔺)	566 ms (+0.03% 🔺)	81 ms (+2.26% 🔺)	646 ms
packages/lexical-rich-text/dist/LexicalRichText.js	39.58 KB (+0.03% 🔺)	792 ms (+0.03% 🔺)	90 ms (-11.74% 🔽)	881 ms
packages/lexical-plain-text/dist/LexicalPlainText.js	39.55 KB (+0.02% 🔺)	792 ms (+0.02% 🔺)	66 ms (+3.98% 🔺)	857 ms