[lexical-table][lexical-playground] Bug Fix: Fix merged cell related edge cases

[etrepum]

Description

• Ensure that TableSelection.getNodes returns each node at most once, which fixes logic errors in merge code when the "first cell" shows up multiple times due to merge
• Account for colSpan and rowSpan in TableSelection.getShape
• Fix some inconsistencies with the implementation of TableCellNode
• Use a heuristic for setting table cell styles when unmerging cells
• Add a mask for TableCellNode.setHeaderStyles so that the row or column style can be explicitly set without affecting the other setting
• Remove restriction on merging "weird shapes" of table cells, all selections are rectangular and can be merged
• Workaround in $computeTableMapSkipCellCheck for non-rectangular tables with rowSpan below the last <tr> in the table. Someone who cares more about these edge cases with what is essentially a bad input should put some more work into this though, since there are likely still other edge cases. The TableMapType and its associated functions (e.g. $computeTableMap) really should get tossed, it's not a good abstraction and it can't represent a non-rectangular table, but because the types are fully transparent it can't be fixed in-place in a backwards compatible way.

/cc @ivailop7

Closes #6599
Closes #4470
Closes #5853
Closes #6605
Closes #6584

Test plan

• Use localhost playground URL or playground URL and merge the top two cells that are already merged
• Unmerge cells

Before

• Prior to this PR it will throw an exception

After

• The merge executes with the correct colSpan/rowSpan (from two cells with colSpan=2 and rowSpan=1 to one cell with colSpan=2 and rowSpan=1)
• The unmerge results in 4 cells with colSpan=1, rowSpan=1

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
lexical	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Sep 7, 2024 4:43am
lexical-playground	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Sep 7, 2024 4:43am

[github-actions]

size-limit report 📦

Path	Size
lexical - cjs	29.66 KB (0%)
lexical - esm	29.48 KB (0%)
@lexical/rich-text - cjs	38.11 KB (+0.11% 🔺)
@lexical/rich-text - esm	31.32 KB (0%)
@lexical/plain-text - cjs	36.65 KB (0%)
@lexical/plain-text - esm	28.66 KB (0%)
@lexical/react - cjs	39.83 KB (0%)
@lexical/react - esm	32.75 KB (0%)

[rilrom]

Great work @etrepum, this is a massive improvement to table cell merging.

[ivailop7]

This is a fantastic step forward! Thank you for putting in the effort in fixing up tables!