Highlight julia-repl code in Markdown specially (#54423)

Fixes #54399 by re-introducing the code seperated out from the styled
Markdown PR at Jameson's request
(#51928 (comment)).

The code itself is modelled after [equivalent code in
OhMyREPL](https://github.com/KristofferC/OhMyREPL.jl/blob/b0071f5ee785a81ca1e69a561586ff270b4dc2bb/src/MarkdownHighlighter.jl#L15-L31).

The new `markdown_julia_prompt` face allows people to make the "prompt"
shown in Markdown code visually distinct, to [avoid confusing it with
the REPL prompt at a
glance](KristofferC/OhMyREPL.jl#100). By way
of example, I make it italic by augmenting my `faces.toml` with

```toml
[markdown]
julia_prompt = { italic = true }
```

stdlib/Markdown/src/Markdown.jl

@@ -45,6 +45,7 @@ const MARKDOWN_FACES = [
     :markdown_h6 => Face(height=1.05, inherit=:markdown_header),
     :markdown_admonition => Face(weight=:bold),
     :markdown_code => Face(inherit=:code),
+    :markdown_julia_prompt => Face(inherit=:repl_prompt_julia),
     :markdown_footnote => Face(inherit=:bright_yellow),
     :markdown_hrule => Face(inherit=:shadow),
     :markdown_inlinecode => Face(inherit=:markdown_code),

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

@@ -111,8 +111,23 @@ function term(io::AnnotIO, md::Header{l}, columns) where l
 end
 
 function term(io::IO, md::Code, columns)
-    code = if md.language ∈ ("", "julia", "julia-repl", "jldoctest")
+    code = if md.language ∈ ("", "julia")
         highlight(md.code)
+    elseif md.language == "julia-repl" || Base.startswith(md.language, "jldoctest")
+        hl = AnnotatedString(md.code)
+        for (; match) in eachmatch(r"(?:^|\n)julia>", hl)
+            StyledStrings.face!(match, :markdown_julia_prompt)
+            afterprompt = match.offset + ncodeunits(match) + 1
+            _, exprend = Meta.parse(md.code, afterprompt, raise = false)
+            highlight!(hl[afterprompt:prevind(md.code, exprend)])
+            if (nextspace = findnext(' ', md.code, exprend)) |> !isnothing
+                nextword = hl[exprend:prevind(hl, nextspace)]
+                if nextword == "ERROR:"
+                    StyledStrings.face!(nextword, :error)
+                end
+            end
+        end
+        hl
     elseif md.language == "styled"
         styled(md.code)
     else