fix: non leading-tabs in markdown content (markedjs#1559)

Only replaces tabs at the beginning of a block construct. Tabs in the
middle of the item are unaffected.

All tests passing. Tabs in both GFM and CommonMark at 100%

fixes markedjs#1559

src/Lexer.js

@@ -115,8 +115,7 @@ export class Lexer {
    */
   lex(src) {
     src = src
-      .replace(/\r\n|\r/g, '\n')
-      .replace(/\t/g, '    ');
+      .replace(/\r\n|\r/g, '\n');
 
     this.blockTokens(src, this.tokens);
 
@@ -135,6 +134,15 @@ export class Lexer {
     if (this.options.pedantic) {
       src = src.replace(/^ +$/gm, '');
     }
+
+    if (this.options.gfm || this.options.pedantic) {
+      src = src.replace(/\t/g, '    ');
+    } else {
+      src = src.replace(/^( *)(\t+)/gm, (_, leading, tabs) => {
+        return leading + '    '.repeat(tabs.length);
+      });
+    }
+
     let token, lastToken, cutSrc, lastParagraphClipped;
 
     while (src) {

src/Tokenizer.js

@@ -187,7 +187,7 @@ export class Tokenizer {
       }
 
       // Get next list item
-      const itemRegex = new RegExp(`^( {0,3}${bull})((?: [^\\n]*)?(?:\\n|$))`);
+      const itemRegex = new RegExp(`^( {0,3}${bull})((?:[\t ][^\\n]*)?(?:\\n|$))`);
 
       // Check if current bullet point can start a new List Item
       while (src) {

src/rules.js

@@ -11,10 +11,10 @@ export const block = {
   newline: /^(?: *(?:\n|$))+/,
   code: /^( {4}[^\n]+(?:\n(?: *(?:\n|$))*)?)+/,
   fences: /^ {0,3}(`{3,}(?=[^`\n]*\n)|~{3,})([^\n]*)\n(?:|([\s\S]*?)\n)(?: {0,3}\1[~`]* *(?=\n|$)|$)/,
-  hr: /^ {0,3}((?:- *){3,}|(?:_ *){3,}|(?:\* *){3,})(?:\n+|$)/,
+  hr: /^ {0,3}((?:-[\t ]*){3,}|(?:_[ \t]*){3,}|(?:\*[ \t]*){3,})(?:\n+|$)/,
   heading: /^ {0,3}(#{1,6})(?=\s|$)(.*)(?:\n+|$)/,
   blockquote: /^( {0,3}> ?(paragraph|[^\n]*)(?:\n|$))+/,
-  list: /^( {0,3}bull)( [^\n]+?)?(?:\n|$)/,
+  list: /^( {0,3}bull)([ \t][^\n]+?)?(?:\n|$)/,
   html: '^ {0,3}(?:' // optional indentation
     + '<(script|pre|style|textarea)[\\s>][\\s\\S]*?(?:</\\1>[^\\n]*\\n+|$)' // (1)
     + '|comment[^\\n]*(\\n+|$)' // (2)