Markdown reader: disallow escaping of ~ and " in markdown_strict

This matches the behavior of the legacy `markdown.pl` as well as what is
described in the manual.

Fixes: #8777

src/Text/Pandoc/Readers/Markdown.hs

@@ -1556,7 +1556,7 @@ escapedChar' = try $ do
   (guardEnabled Ext_all_symbols_escapable >> satisfy (not . isAlphaNum))
      <|> (guardEnabled Ext_angle_brackets_escapable >>
             oneOf "\\`*_{}[]()>#+-.!~\"<>")
-     <|> oneOf "\\`*_{}[]()>#+-.!~\""
+     <|> oneOf "\\`*_{}[]()>#+-.!"
 
 escapedNewline :: PandocMonad m => MarkdownParser m (F Inlines)
 escapedNewline = do

test/command/8777.md

@@ -0,0 +1,8 @@
+# Backslash escapes in markdown_strict
+
+```
+% pandoc --from=markdown_strict -t html
+These are not escaped: \~ \"
+^D
+<p>These are not escaped: \~ \"</p>
+```