Improve Markdown Syntax Highlighting of Nested Lists (#14634)

**bug**
Indented lists in markdown stop highlighting after four indents, even if they are nested.

**Fix**
Allow lists to also match the start of the line instead of the anchor point in their while clause. This ensure that each list element can consume up to three spaces at each level.

extensions/markdown/syntaxes/markdown.tmLanguage

@@ -476,7 +476,7 @@
 								</dict>
 							</array>
 							<key>while</key>
-							<string>\G([ ]{4}|\t|$)</string>
+							<string>(^|\G)([ ]{4}|\t)</string>
 						</dict>
 						<dict>
 							<key>begin</key>
@@ -503,7 +503,7 @@
 								</dict>
 							</array>
 							<key>while</key>
-							<string>\G([ ]{4}|\t|$)</string>
+							<string>(^|\G)([ ]{4}|\t)</string>
 						</dict>
 					</array>
 				</dict>