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fix(postgrest): read a JSON path ending in an array index under the key PostgREST answers with (#1915)

## Summary

A typed JSON path that ends in an array index always reads back `null`
from a `PostgrestPartialRow`.

```dart
final firstTag = Orders.metadata.jsonObject('tags').jsonText('0');

final row = await client.table(Orders.table).selectOnly([firstTag]).single();
row.read(firstTag); // null, although the response holds {"tags": "urgent"}
```

`jsonText` and `jsonObject` use the path segment they were given as the
response key, so the row is read under `'0'`. PostgREST does not name
the value after an index. It uses the last key of the path, and the
column name when the path has no key ([`lastJsonKey` in
`Plan.hs`](https://github.com/PostgREST/postgrest/blob/main/src/library/PostgREST/Plan.hs#L495-L503)):

```haskell
-- `select=data->1->mycol->>2`, we need to show the result as [ {"mycol": ..}, {"mycol": ..} ]
-- `select=data->3`, we need to show the result as [ {"data": ..}, {"data": ..} ]
```

Nothing throws, because the entry was selected, so the lookup just
misses.

The fix keeps the key of the expression the path was applied to when the
segment is an index. An index is an optional `-` followed by digits, the
same rule PostgREST's parser applies ([`pJIdx` in
`QueryParams.hs`](https://github.com/PostgREST/postgrest/blob/main/src/library/PostgREST/ApiRequest/QueryParams.hs)).
All `jsonText` and `jsonObject` implementations go through one helper:
stored columns, computed fields, chained JSON paths and derived
expressions.

| Expression | Sent | Read under, before | Read under, after |
| -- | -- | -- | -- |
| `metadata.jsonText('gift')` | `metadata->>gift` | `gift` | `gift` |
| `metadata.jsonText('0')` | `metadata->>0` | `0` | `metadata` |
| `metadata.jsonObject('tags').jsonText('0')` | `metadata->tags->>0` |
`0` | `tags` |
| `metadata.jsonObject('items').jsonObject('-1')` |
`metadata->items->-1` | `-1` | `items` |
|
`metadata.jsonObject('items')... (continued)

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