* lisp/language/indian.el ("Siddham"): New language environment.
Add composition rules for Siddham. Add sample text and input
method.
* lisp/international/fontset.el (script-representative-chars)
(setup-default-fontset): Support Siddham.
* lisp/leim/quail/indian.el ("siddham"): New input method.
* etc/HELLO: Add a Siddham greeting.
* etc/NEWS: Announce the new language environment and its
input method. (Bug#55350)