Change `Field.__repr__` to match representation in `pystencils.fields`

__repr__ of Field should look like

```
y: uint16_t[246,704,558]
x(3): float[20,40]
```

to match representation when using `pystencils.fields`
8 jobs for Field.__repr__ in 23 minutes and 2 seconds (queued for 4 minutes and 57 seconds)
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