Add support for Tai Tham script

Add support for Tai Tham script

* lisp/international/fontset.el (setup-default-fontset)
(script-representative-chars): Add tai-tham.
* lisp/language/thai.el ("Northern Thai"): New language
environment.  Patch by Richard Wordingham
<richard.wordingham@ntlworld.com>.  Set
'composition-function-table' for the Tai Tham block.  Original
code by Richard Wordingham <richard.wordingham@ntlworld.com>.

* etc/HELLO: Add Northern Thai greeting.
* etc/NEWS: Announce addition of Northern Thai language environment.