sql: introduce literals for DECIMAL

Part of #6356

@TarantoolBot document
Title: Literals for INTEGER, DECIMAL and DOUBLE

The rules for parsing numeric values have changed:
1) a value consisting of digits without decimal point and exponent will
be parsed as INTEGER;
2) a value consisting of digits and a decimal point will be parsed as
DECIMAL;
3) a value consisting of digits, containing an exponent and possibly
containing a decimal point, will be parsed as DOUBLE.