Fix bug 27 of #17340 (#19433)

Fixes silent disappearance of Markdown (pseudo-)link when it's detected as
unsafe protocol. Now it will be converted to plain text in spirit of
[the specification](https://spec.commonmark.org/0.30/#links).
For that sake the check for protocol is added to rst.nim also.

lib/packages/docutils/rstgen.nim

@@ -40,7 +40,7 @@
 ##   can be done by simply searching for [footnoteName].
 
 import strutils, os, hashes, strtabs, rstast, rst, highlite, tables, sequtils,
-  algorithm, parseutils, std/strbasics, strscans
+  algorithm, parseutils, std/strbasics
 
 import ../../std/private/since
 
@@ -826,17 +826,6 @@ proc renderOverline(d: PDoc, n: PRstNode, result: var string) =
                    "\\rstov$4[$5]{$3}$2\n", [$n.level,
         rstnodeToRefname(n).idS, tmp, $chr(n.level - 1 + ord('A')), tocName])
 
-
-proc safeProtocol(linkStr: var string) =
-  var protocol = ""
-  if scanf(linkStr, "$w:", protocol):
-    # if it has a protocol at all, ensure that it's not 'javascript:' or worse:
-    if cmpIgnoreCase(protocol, "http") == 0 or cmpIgnoreCase(protocol, "https") == 0 or
-        cmpIgnoreCase(protocol, "ftp") == 0:
-      discard "it's fine"
-    else:
-      linkStr = ""
-
 proc renderTocEntry(d: PDoc, e: TocEntry, result: var string) =
   dispA(d.target, result,
     "<li><a class=\"reference\" id=\"$1_toc\" href=\"#$1\">$2</a></li>\n",
@@ -901,7 +890,7 @@ proc renderImage(d: PDoc, n: PRstNode, result: var string) =
 
   # support for `:target:` links for images:
   var target = esc(d.target, getFieldValue(n, "target").strip(), escMode=emUrl)
-  safeProtocol(target)
+  discard safeProtocol(target)
 
   if target.len > 0:
     # `htmlOut` needs to be of the following format for link to work for images:
@@ -1205,7 +1194,7 @@ proc renderHyperlink(d: PDoc, text, link: PRstNode, result: var string,
     d.escMode = emUrl
     renderRstToOut(d, link, linkStr)
     d.escMode = mode
-  safeProtocol(linkStr)
+  discard safeProtocol(linkStr)
   var textStr = ""
   renderRstToOut(d, text, textStr)
   let nimDocStr = if nimdoc: " nimdoc" else: ""