Extend parser.jj in calcite, to support the operator '<=>'

[徐仁和]

Hi calcite community.

In my case, we need calcite's parser support operator '<=>' of mysql.

Link:
https://dev.mysql.com/doc/refman/8.0/en/comparison-operators.html#operator_equal-to

I debug it, I find I need to modify the Parser.jj in calcite, to define a
new token for '<=>'.

So, could I expand the Parser.jj of calcite, and support this new operator.

Best,
XuRenhe