Improve import/export functions for sheetjs with x-spreadsheet

[ThibautSF]

Pull Request for #2410

Integration of improves import and export functions in x-spreadsheet demo from myliang/x-spreadsheet#419.

Allow optional arguments to support new QoL functionalities (default to previous behaviour)

• keep merged cells between formats
• choose to keep formulas (as string ie '=SUM(...)') or formula result (default behaviour)

Note: Incorporate suggestions by @Liyaa

[SheetJSDev]

Thanks for looking into this!

Is there a reason not to enable keepMerges? (I think the option should be removed and it should always keep merges)

Is there a way to distinguish the text =1+1 from the formula =1+1? If not, the formula logic is fine.

Ideally we'd merge one commit, so please squash down

[ThibautSF]

Is there a reason not to enable keepMerges? (I think the option should be removed and it should always keep merges)

I did that mainly to have the least interferences on the function behaviour. But indeed it can be removed (myself I never use with "false" value).

Is there a way to distinguish the text =1+1 from the formula =1+1? If not, the formula logic is fine.

I have some knowledge about x-spreadsheet due to developments around this lib. And while SheetJS has the cell.f attribute which helps to make the difference in stox(); on the other hand, I didn't see that kind of information in data from x-spreadsheet.
In fact, inside the lib, functions are calc based on the equal sign presence.

Ideally we'd merge one commit, so please squash down

Ok I will do that (after modifications for keepMerges option if you confirm it's needed)

[SheetJSDev]

If x-spreadsheet detects functions based on leading =, then get rid of the option and always keep formulae. It would be better for stox to manually loop the worksheet instead of using sheet_to_json, but that can be fixed another time.

[SheetJSDev]

5c02c81