feat: replace fence indent space with ZWSP to preserve indent information (#10)

Author: galaxian85

lib/rules_block/fence.js

@@ -91,7 +91,7 @@ module.exports = function fence(state, startLine, endLine, silent) {
 
   token         = state.push('fence', 'code', 0);
   token.info    = params;
-  token.content = state.getLines(startLine + 1, nextLine, len, true);
+  token.content = state.getLines(startLine + 1, nextLine, len, true, true);
   token.markup  = markup;
   token.map     = [ startLine, state.line ];
   token.position = originalPos;

lib/rules_block/state_block.js

@@ -176,10 +176,12 @@ StateBlock.prototype.skipCharsBack = function skipCharsBack(pos, code, min) {
   return pos;
 };
 
+const ZWSP = '\u200B'; // zero width space
 // cut lines range from source.
-StateBlock.prototype.getLines = function getLines(begin, end, indent, keepLastLF) {
+StateBlock.prototype.getLines = function getLines(begin, end, indent, keepLastLF, replaceIndentSpaceWithZWSP) {
   var i, lineIndent, ch, first, last, queue, lineStart,
-      line = begin;
+      line = begin,
+      replaceIndentSpaceWithZWSP = replaceIndentSpaceWithZWSP || false;
 
   if (begin >= end) {
     return '';
@@ -224,6 +226,10 @@ StateBlock.prototype.getLines = function getLines(begin, end, indent, keepLastLF
     } else {
       queue[i] = this.src.slice(first, last);
     }
+
+    if (replaceIndentSpaceWithZWSP) {
+      queue[i] = new Array(lineIndent + 1).join(ZWSP) + queue[i];
+    }
   }
 
   return queue.join('');