org-babel: Declare :async header argument in the backends that support it

* lisp/ob-R.el (org-babel-header-args:R):
* lisp/ob-python.el (org-babel-header-args:python): Declare async
header argument.
* lisp/ob-shell.el (org-babel-header-args:shell): Add variable.
(org-babel-shell-initialize): Propagate base
`org-babel-header-args:shell' to `org-babel-header-args:<shell-name>'.

Without declaration, org-lint would not recognize :async argument.

lisp/ob-R.el

@@ -64,6 +64,7 @@
     (colormodel		 . :any)
     (useDingbats	 . :any)
     (horizontal		 . :any)
+    (async               . ((yes no)))
     (results             . ((file list vector table scalar verbatim)
 			    (raw html latex org code pp drawer)
 			    (replace silent none append prepend)

lisp/ob-python.el

@@ -43,7 +43,8 @@
 
 (defconst org-babel-header-args:python
   '((return . :any)
-    (python . :any))
+    (python . :any)
+    (async . ((yes no))))
   "Python-specific header arguments.")
 
 (defcustom org-babel-python-command 'auto

lisp/ob-shell.el

@@ -45,6 +45,11 @@
 (declare-function orgtbl-to-generic "org-table" (table params))
 
 (defvar org-babel-default-header-args:shell '())
+
+(defconst org-babel-header-args:shell
+  '((async               . ((yes no))))
+  "Shell-specific header arguments.")
+
 (defvar org-babel-shell-names)
 
 (defconst org-babel-shell-set-prompt-commands
@@ -86,6 +91,9 @@ variables."
 	      name))
     (funcall (if (fboundp 'defvar-1) #'defvar-1 #'set) ;Emacs-29
              (intern (concat "org-babel-default-header-args:" name))
+             nil)
+    (funcall (if (fboundp 'defvar-1) #'defvar-1 #'set) ;Emacs-29
+             (intern (concat "org-babel-header-args:" name))
              nil)))
 
 (defcustom org-babel-shell-names