Add new customizable phonetic Tamil input method

Add new customizable phonetic Tamil input method

* lisp/language/indian.el ("Tamil"): Change the default input method
of the Tamil language environment to the new input method.

* lisp/leim/quail/indian.el
(quail-tamil-itrans-compute-syllable-table): New function extracted
from...
(quail-tamil-itrans-syllable-table): ... here.  Use the above
function.
(quail-tamil-itrans--consonant-order): Auxiliary variable for the
above function.
(quail-tamil-itrans-compute-signs-table): Add new VARIOUS argument.
(quail-tamil-itrans-various-signs-and-digits-table)
(quail-tamil-itrans-various-signs-table): Adjust call to the above
function.

("tamil-phonetic"): Add new input method.
(tamil-input): New group for the input method.
(tamil-translation-rules): New defcustom for the input method to
change the translation rules.

(tamil--syllable-table, tamil--signs-table, tamil--hashtables)
(tamil--vowel-signs): Internal variables used by the input method.
(tamil--setter, tamil--make-tables)
(tamil--update-quail-rules): Internal functions for the input method.
(bug#56323)

* etc/NEWS: Announce the new input method.