Excel: support text style hints (e.g. bold / italic) in formatted number values

Currently it is not possible to hint that the value should be output as bold / italic / underlined. Potentially coloured etc.

Our use case is making the first line of a spilling function (header line) bold by default.

We would like to see this support added to FNVs to enrich the users' experience.

https://learn.microsoft.com/en-us/office/dev/add-ins/excel/custom-functions-data-types-concepts#output-a-formatted-number-value