Normalise "R" string prefix to lower case

[felix-hilden]

Describe the style change
I'd like to normalise raw strings with a capital R, like R"flowers" to lower case: r"flowers".

Additional context
Currently our reasoning for keeping "R" is that a syntax highlighter called MagicPython differentiates between the two. And because it is the default in GitHub and VS Code, we should be careful about changing it.

Currently though, I think Pygments is the standard syntax highlighter. I don't know much about MagicPython, but it hasn't been updated since 2020. 1.3k GH stars is a lot, yes, but all of these signs show to me that maybe we shouldn't make an exception here anymore.

[felix-hilden]

Richard wrote on Discord, that VS Code still uses MagicPython. Doesn't bother me personally, but this isn't as trivial as I thought at first. Style-wise I still dislike the capital "R", and this would be a neat change to land with #2297.

[ichard26]

I was going to reject this suggestion... however, I did some digging and it looks like GitHub stopped using MagicPython as their syntax highlighter! Visual Studio Code still uses MagicPython and I still don't think it's worth changing this, but there is some room for debate.