Flexibility updating table's cells. (#4027)

Very often we need to work with tables (say extracted from unstructured data or some
kind of final report, timeseries, and the like).

It's inevitable we will be having columns that we can't know beforehand what their names
will be, or how many.

Also, we may end up with certain cells having values we may want to remove as we explore.

Here, `update cells` fundamentally goes over every cell in the table coming in and updates
the cell's contents with the output of the block passed. Basic example here:

```
> [

    [   ty1,       t2,       ty];

    [     1,        a, $nothing]
    [(wrap), (0..<10),      1Mb]
    [    1s,     ({}),  1000000]
    [ $true,   $false,   ([[]])]

] | update cells { describe }

───┬───────────────────────┬───────────────────────────┬──────────
 # │          ty1          │            t2             │    ty
───┼───────────────────────┼───────────────────────────┼──────────
 0 │ integer               │ string                    │ nothing
 1 │ row Column(table of ) │ range[[integer, integer)] │ filesize
 2 │ string                │ nothing                   │ integer
 3 │ boolean               │ boolean                   │ table of
───┴───────────────────────┴───────────────────────────┴──────────
```

and another one (in the examples) for cases, say we have a timeseries table generated and
we want to remove the zeros and have empty strings and save it out to something like CSV.

```
> [
    [2021-04-16, 2021-06-10, 2021-09-18, 2021-10-15, 2021-11-16, 2021-11-17, 2021-11-18];
    [        37,          0,          0,          0,         37,          0,          0]
] | update cells {|value| i
  if ($value | into int) == 0 {
    ""
  } {
    $value
  }
}

───┬────────────┬────────────┬────────────┬────────────┬────────────┬────────────┬────────────
 # │ 2021-04-16 │ 2021-06-10 │ 2021-09-18 │ 2021-10-15 │ 2021-11-16 │ 2021-11-17 │ 2021-11-18
───┼────────────┼────────────┼────────────┼────────────┼────────────┼────────────┼────────────
 0 │         37 │            │            │            │         37 │            │
───┴────────────┴────────────┴────────────┴────────────┴────────────┴────────────┴────────────
```

crates/nu-command/src/default_context.rs

@@ -134,6 +134,7 @@ pub fn create_default_context(interactive: bool) -> Result<EvaluationContext, Bo
             whole_stream_command(Select),
             whole_stream_command(Get),
             whole_stream_command(Update),
+            whole_stream_command(UpdateCells),
             whole_stream_command(Insert),
             whole_stream_command(Into),
             whole_stream_command(IntoBinary),