define * for rich text concatenation

[ryofurue]

Feature description

I've found it useful to define * for Makie.RichText. I thought it would be a little nice addition. See below for an example to illustrate how it can be useful.

I sometimes write functions that call Makie functions and in my functions, I don't know the type of string arguments. They can be just Strings or Makie.RichTexts:

using CairoMakie
function Base.:*(x::Makie.RichText, y::AbstractString)
  rich(x, y)
end
function Base.:*(x::AbstractString, y::Makie.RichText)
  rich(x, y)
end

function plotsomething(title)
  mytitle = "hello, " * title * "!"
  # plot something using mytitle
end

plotsomething("x")
plotsomething(rich("x", superscript("y")))