feat: add drawing pass-through support for unsupported elements

[rpg600]

feat: add drawing pass-through support for unsupported elements

Add opt-in pass-through mechanism to preserve unsupported drawing
elements (shapes, textboxes etc) during read/write operations.

• Add enableDrawingPassThrough property to BaseReader
• Store original drawing XML when pass-through is enabled
• Reuse stored XML in Writer instead of regenerating
• Disabled by default for backward compatibility

This allows preservation of Excel elements not yet supported by
PhpSpreadsheet without requiring full implementation.

Currently, the pass-through mechanism works as follows:

1. Reader: setEnableDrawingPassThrough(true) stores the original drawing XML
2. Writer: Automatically uses the stored XML if:
   • The drawing collection is empty (no drawings added programmatically)
   • AND the unparsed data contains the original XML

I'm open to adding an explicit Writer flag if you believe it provides better clarity and control.

This addresses existing issues : #4681, #4037, #2621

[oleibman]

I am still evaluating whether this is a good idea or not. Will it work if you add or remove a row or a column on the sheet? Add a comment when there weren't any before, and when there were? Remove a comment?

Your commentary says that you are fixing 3 open issues. Are there tests for each of those?