[IMP] formulas: add support for '+' as first character for formulas

[Rachico]

This commit adds support for using the plus symbol to add formulas. If you start with a "+" sign, it's automatically interpreted as a "=" and the cell content is saved with "=". This behaviour was inspired by other spreadsheet applications that also support this feature.

Task: 4460183

review checklist

• [ ] feature is organized in plugin, or UI components
• [ ] support of duplicate sheet (deep copy)
• [ ] in model/core: ranges are Range object, and can be adapted (adaptRanges)
• [ ] in model/UI: ranges are strings (to show the user)
• [ ] undo-able commands (uses this.history.update)
• [ ] multiuser-able commands (has inverse commands and transformations where needed)
• [ ] new/updated/removed commands are documented
• [ ] exportable in excel
• [ ] translations (_t("qmsdf %s", abc))
• [ ] unit tested
• [ ] clean commented code
• [ ] track breaking changes
• [ ] doc is rebuild (npm run doc)
• [ ] status is correct in Odoo

[robodoo]