Add Storage.AccountStorageFormat()

[fxamacker]

Updates #3584

This PR exports runtime.StorageFormat and adds Storage.AccountStorageFormat().

Storage.AccountStorageFormat() returns:

• either StorageFormatV1 or StorageFormatV2 for existing accounts
• StorageFormatUnknown for non-existing accounts.

Thanks @turbolent for this idea!

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[turbolent]

Great work!