Support for TOP expressions

[mlkammer]

It would be great if JSQLParser could also parse TOP expressions.

Currently only two variants are supported as a TOP clause and these will parse successfully:

1. Constant value, example: SELECT TOP (2) * FROM MyTable
2. JDBC parameter, example: SELECT TOP (?) * FROM MyTable

More complex expressions are currently not supported and give a ParseException. Example queries that fail to be parsed:

• SELECT TOP (? + 1) * FROM MyTable
• SELECT TOP (CAST(? AS INT)) * FROM MyTable

I feel like it could be relatively simple to implement this, by letting net.sf.jsqlparser.statement.select.Top have a method getRowCountExpression which returns a scalar expression as a net.sf.jsqlparser.expression.Expression object.

On a side note: the JSQLParser library is greatly appreciated by us. Thanks to the authors!

[mlkammer]

Great work, Tobias! Will check it out soon.