Follow contemporary practices in the Tamil input methods

Follow contemporary practices in the Tamil input methods

* lisp/language/ind-util.el (indian-tml-base-digits-table): New table
to translate digits.
(indian-tml-base-table): Don't translate digits.
(indian-tml-itrans-digits-v5-hash): Hashtable that translates digits.

* lisp/leim/quail/indian.el (quail-tamil-itrans-compute-signs-table)
(quail-tamil-itrans-various-signs-table)
(quail-tamil-itrans-various-signs-and-digits-table): Separate out the
digits from the signs table using the function
'quail-tamil-itrans-compute-signs-table'.
(tamil-itrans-digits, tamil-inscript-digits): New input methods that
translates digits.
(tamil-itrans, tamil-inscript): Fix table inclusion.

* etc/NEWS: Announce the change.

Fixes bug#50143.

Copyright-paperwork-exempt: yes