Add support for precomputed result with formula

[xperseguers]

One of the limitation of the current implementation, is that formula written don't have any value visible until opened in Excel (see https://github.com/doPhp/excel/blob/main/README.md#formulas-written-but-no-values-readable).

It turns out LibXL supports this, e.g., thanks to writeFormulaNum since v3.7.0.

Somehow it never was implemented as this project is mainly into "survival mode" (but many thanks for that fork making it compatible with PHP 8! I'm currently using with PHP 8.3).

I can prepare a PR, but right now, if someone's interested, that's my very first work on a PHP extension:

xperseguers@5944032

And in short what it does:

Twitted (well "X-ed") about it: https://x.com/xperseguers/status/1828841162872107025

[xperseguers]

Ticket will be closed with #30