patch 8.2.2343: Vim9: return type of readfile() is any

Problem:    Vim9: return type of readfile() is any.
Solution:   Add readblob() so that readfile() can be expected to always
            return a list of strings. (closes #7671)

runtime/doc/usr_41.txt

@@ -820,6 +820,7 @@ System functions and manipulation of files:
 	setenv()		set an environment variable
 	hostname()		name of the system
 	readfile()		read a file into a List of lines
+	readblob()		read a file into a Blob
 	readdir()		get a List of file names in a directory
 	readdirex()		get a List of file information in a directory
 	writefile()		write a List of lines or Blob into a file