Support octal integer constants

[aleofreddi]

Is your feature request related to a problem? Please describe.
For the sake of compability with the rest of the world, TimEL should be able to understand octal constants.

Describe the solution you'd like
Add octal support:

fragment
IntegerConstant
    :   OctalConstant

fragment
OctalConstant
    :   '0' OctalDigit*
    ;

Describe alternatives you've considered
/

Additional context
/

[aleofreddi]

Fixed