Custom operator for Groovy 3

[孙 岚]

Hi all,

         The new parser support defining custom operator now. The initial 
implementation is on the customOperator branch of 
groovy-parser(https://github.com/danielsun1106/groovy-parser/tree/customOperator).

          Some examples can be found at 
https://github.com/danielsun1106/groovy-parser/blob/c3259607c4ee0cda6e5619bed35d0674e26f78aa/src/test/resources/core/CustomOperator_01x.groovy

           In order to make the custom operator have more priorities, the 
custom operators have lower priority by default, and the priority can be 
changed by using parentheses.

           Any thoughts?

Cheers,
Daniel.Sun