Add support for the Lepcha script (bug#55869)

Add support for the Lepcha script (bug#55869)

* lisp/language/indian.el ("Lepcha"): New language environment.
Add composition rules for Lepcha. Add sample text and input method.
* lisp/language/misc-lang.el ("Kharoshthi"): Fix whitespace.
* lisp/international/fontset.el (script-representative-chars)
(setup-default-fontset): Support Lepcha.
* lisp/leim/quail/indian.el ("lepcha"): New input method.

* etc/HELLO: Add a Lepcha greeting.
* etc/NEWS: Announce the new language environment.