Color todo admonitions in the REPL (JuliaLang#54957)

This patch adds magenta coloring for `!!! todo` admonitions in addition
to the existing styling of `danger`, `warning`, `info`, `note`, `tip`,
and `compat` admonitions. This is useful if you want to leave some more
colorful todo notes in docstrings, for example.

Accompanying PR for Documenter to render these in HTML and PDF docs:
JuliaDocs/Documenter.jl#2526.

Author: fredrikekre

stdlib/Markdown/src/render/terminal/render.jl

@@ -39,6 +39,8 @@ function term(io::IO, md::Admonition, columns)
         Symbol(md.category)
     elseif md.category == "compat"
         :bright_cyan
+    elseif md.category == "todo"
+        :magenta
     else
         :default
     end