[lexical-utils] Bug Fix: Refactor markSelection for getDOMSlot and not using updateDOM

[etrepum]

Description

The usage of updateDOM in markSelection seemed to create DOM churn in certain circumstances for CJK composition inputs which affected user input and selection. Refactoring the function to instead use the invariant that if updateDOM did return true, then the identity of the rendered DOM element would've changed.

While I was in there I also refactored the range code to support the getDOMSlot API. We don't currently have any code that offsets the first child, but if we did then we'd have another bug to fix here.

Closes #6947
Closes #6946

Test plan

Should be covered by existing SelectionAlwaysOnDisplay test coverage

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Name	Status	Preview	Comments	Updated (UTC)
lexical	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Dec 14, 2024 6:31pm
lexical-playground	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Dec 14, 2024 6:31pm

[github-actions]

size-limit report 📦

Path	Size
lexical - cjs	31.2 KB (0%)
lexical - esm	31.04 KB (0%)
@lexical/rich-text - cjs	40.16 KB (0%)
@lexical/rich-text - esm	32.95 KB (0%)
@lexical/plain-text - cjs	38.77 KB (0%)
@lexical/plain-text - esm	30.16 KB (0%)
@lexical/react - cjs	42.05 KB (0%)
@lexical/react - esm	34.24 KB (0%)