Port the original TableCellSelection plugin and merge the logic into existing SelectionPlugin. Now the table selection become part of editor core, and no need to add any plugin explicitly for it.

Expose the following new types:

• TableCellCoordinate: {row, col}
• TableSelectionInfo: Store current table selection info, can be incomplete since user is still moving mouse
• ParsedTable: a 2D array for TD or its span state

Create the following internal util functions:

• findTableCellElement: Try to find a TD/TH element from the given row and col number from the given parsed table
• findLastedCoInMergedCell: Try to find the last logic cell of a merged table cell
• findCoordinate: Find coordinate of a given element from a parsed table
• normalizePos: Find the deepest position of the given position (aka Position.normalize() in old code)

Add a IgnoredPluginNames const in EditorAdapter to aways ignore old TableCellSelection plugin since it will be conflict with the new one

Also handle keyboard event for the following cases:

• Up/Down in table: move focus into the cell above/below
• Shift+Up/Down in table: select current cell and the cell above/below. If there is already table selection, adjust the selection according to the key
• Shift+Left/Right in table: select current cell and the cell left/right. If there is already table selection, adjust the selection according to the key

I didn't handle TAB key in this change since it is not directly related to table selection. We will handle it in future changes.

Fix a bug in getDOMSelection when check isReverted.
Fix #861