Add support for the Syloti Nagri script

Add support for the Syloti Nagri script

* lisp/language/indian.el ("Syloti Nagri"): New language environment.
Add composition rules for Syloti Nagri.  Add sample text and input
method.
Improve composition rules for Kaithi, Tirhuta, Sharada and Siddham.
* lisp/international/fontset.el (script-representative-chars)
(setup-default-fontset): Support Syloti Nagri.
* lisp/leim/quail/indian.el ("syloti-nagri"): New input method.

* etc/HELLO: Add two Syloti Nagri greetings.
* etc/NEWS: Announce the new language environment and its
input method.