[8.x] Add a simple Str::repeat() helper function

[JonaLoeffler]

This PR adds a simple repeat() function to the Str and Stringable classes.

Given most other string manipulation methods are present in these classes, one might also expect to find the str_repeat() functionality.

[GrahamCampbell]

This seems quite unnecessary IMO.

[JonaLoeffler]

I am currently using a longer Stringable expression and because there currently is no repeat() I had to break that chain an manually call str_repeat() making the code harder to read.

A straightforward solution is to add macros to both Str and Stringable that achieve exactly the same behaviour, but I thought this might actually be useful to others.