@debug the next REPL line before evaling it

Add a `@debug` expression within DocTests.eval_repl to show which line is
about to be evaluated.
This was requested in
#2013

src/DocTests.jl

@@ -241,6 +241,7 @@ end
 
 function eval_repl(block, sandbox, meta::Dict, doc::Documenter.Document, page)
     for (input, output) in repl_splitter(block.code)
+        @debug "Evaluating REPL line" input output
         result = Result(block, input, output, meta[:CurrentFile])
         for (ex, str) in Documenter.parseblock(input, doc, page; keywords = false, raise=false)
             # Input containing a semi-colon gets suppressed in the final output.