Add "let" operator

The `let` operator allows users to assign variables to be used in queries.  Example follows.

```
let n = 10;  // number
let place = "Dallas";  // string
let cutoff = ago(62d); // datetime 
Events 
| where timestamp > cutoff 
    and city == place 
| take n
```

Kusto docs:

https://learn.microsoft.com/en-us/azure/data-explorer/kusto/query/let-statement