Add more known function invocations

[phiresky]

e.g.

const distinct = (arg: s.Expression<any>) => new s.KnownFunctionInvocation("distinct", [arg], arg.type);
const min = (arg: s.Expression<any>) => new s.KnownFunctionInvocation("min", [arg], arg.type);
const max = (arg: s.Expression<any>) => new s.KnownFunctionInvocation("max", [arg], arg.type);
const nullif = (arg: s.Expression<any>, arg2: s.Expression<any>) => new s.KnownFunctionInvocation("nullif", [arg, arg2], arg.type);

[phiresky]

also abs

[hediet]

Done.