fix: the idx of a pair inside or inside pair is not calculated correctly

[ac10n]

Closes #2927
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Release Note Draft Snippet

This is a breaking change in (In API behaviour, not the API itself).
Previously, when Michelson Data was converted to javascript objects (For instance, by calling storage method of a contract abstraction), in the following scenario we would have an unexpected behaviour:

1. The javascript representation has nested objects, like having a pair, inside an or inside another pair
2. Some of the pair's items don't have annots.

In this case, the the resulting javascript object would look like this

{
  '0': 'firstValue',
  '1': 'secondValue,
  '2': {
   '2': 'thirdValue',
   '3': 'fourthValue'
  }

Please note the the inner object's field numbers depend on the object's location in it's parent, and starts with '2' in this example. With this PR, this changes to:

{
  '0': 'firstValue',
  '1': 'secondValue,
  '2': {
   '0': 'thirdValue',
   '1': 'fourthValue'
  }

If your dApp code depends on the old behaviour (or changing it requires too much effort, you can call Tezos.setFieldNumberingStrategy('Legacy') to keep thew previous behaviour. But please note that in a future release, this option might be removed.

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