improve random strings

[JanMarvin]

This is a bit tricky and I'd be open to suggestions how to improve this or if we are fine with this. I needed it, to run a test and it works well. Therefore ... another suggestion must be better :)

This approach does not alter the seed when calling random_string(), even with strings of random sizes. I applied these in a row, but ran into a problem, where I actually wanted the seed to be altered when calling it in openxlsx2, therefore I've added a way for openxlsx2 to handle such cases.

Once random_string()is called:

1. We backup the current seed and check for .openxlsx2.seed, this is the seed we want to use.
2. If none is found, we create one ourself.
3. Afterwards we use stringi to create a random string for us.
4. We store the altered string as .openxlsx2.seed therefore once we call random_string() again, it will pick up this seed.
5. We restore the previous global seed as if it was never altered.

[JanMarvin]

The onus is, we pollute the .GlobalEnv with our seed and if we want to set the seed for random_seed() we have to remove .openxlsx2.seed previously.

[JanMarvin]

I use this in #280

[jmbarbone]

withr::with_seed() also checks seed type, which we may want to do as well: https://github.com/r-lib/withr/blob/main/R/seed.R

[JanMarvin]

fixed the tests

[JanMarvin]

We could add a seed counter option. We set and increase in random_string() and use set.seed() with it. That would be the slimmest approach, but personally I'm fine with both. The current in this PR and the sketched idea as well. I see if I have time to add this later today, otherwise I'll merge with a to-do note. It's a minor function without much use at the moment. Unless someone wants to add styles or sparklines it isn't triggered at all.

[JanMarvin]

Scratch that last comment. Check complains therefore I'll change it.

Found the following assignments to the global environment:
File ‘openxlsx2/R/utils.R’:
  assign(".openxlsx2.seed", .Random.seed, globalenv())