Simplify markdown paragraph logic (#18531)

* Simplify markdown paragraph logic

* Fix for alt headers

extensions/markdown/syntaxes/markdown.tmLanguage

@@ -48,10 +48,6 @@
 				</dict>
 				<dict>
 					<key>include</key>
-
-
-
-
 					<string>#fenced_code_block_css</string>
 				</dict>
 				<dict>
@@ -567,10 +563,7 @@
 						</dict>
 					</array>
 					<key>while</key>
-					<!-- seperator    [ ]{0,3}([-*_][ ]{0,2}\2){2,}[ \t]*$\n? -->
-					<!-- list         [ ]{0,3}[*+-]([ ]{1,3}|\t) -->
-					<!-- both are folded together in the expression below -->
-					<string>(^|\G)(?!\s*$|#|[ ]{0,3}((([*_][ ]{0,2}\2){2,}[ \t]*$\n?)|([*+-]([ ]{1,3}|\t)))|\s*\[.+?\]:|&gt;)</string>
+					<string>(^|\G)((?=\s*[-=]{3,}\s*$)|[ ]{4,}(?=\S))</string>
 				</dict>
 				<key>fenced_code_block_css</key>
 				<dict>
@@ -968,7 +961,6 @@
 							<string>punctuation.definition.markdown</string>
 						</dict>
 					</dict>
-
 					<key>patterns</key>
 					<array>
 						<dict>