Decimal literal support

[karteekmurthys]

Add support for parsing decimal literals like:

presto> select decimal'1.2';
 _col0 
-------
 1.2   
(1 row)
presto> select * from (values decimal '1234567890001234.56789', decimal '2234567890001234.12345');
         _col0
------------------------
 1234567890001234.56789
 2234567890001234.12345

[stale[bot]]

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