fix(help): :helptags on CRLF helpfiles

Problem:
The vimdoc parser does not treat "\r" as whitespace, so in a CRLF
helpfile the codeblock rule fails and "*" pairs inside examples become
tags. On Windows this generates duplicate "." and "/" tags:

    D:/a/neovim/neovim/build/bin/nvim.exe -u NONE -i NONE -e --headless -c "helptags ++t doc" -c "exe 'cquit' !empty(v:errmsg)""
    Error in command line:
    E154: Duplicate tag "." in gui.txt and repeat.txt
    E154: Duplicate tag "/" in pattern.txt and usr_08.txt
    E154: Duplicate tag "/" in usr_08.txt and pattern.txt

Note: The old C parser was unaffected because it read helpfiles in text
mode (`os_fopen(…, "r")`).

Solution:
Strip "\r" before parsing.

runtime/lua/vim/_core/help.lua

@@ -433,6 +433,8 @@ local function extract_tags(tags, file, name)
     return
   end
   --- @cast source string
+  -- The grammar treats "\r" as part of a word, so CRLF files would yield bogus tags.
+  source = source:gsub('\r\n', '\n')
 
   local query = ts.query.parse('vimdoc', '(tag (word) @tagname)')
   local parser = ts.get_string_parser(source, 'vimdoc')