Fix implicit cast operator for JsonNumber

[kasiabulat]

Implicit cast operator for JsonNumber can throw an exception, which is against the FDG:
https://docs.microsoft.com/en-us/dotnet/standard/design-guidelines/operator-overloads#conversion-operators. We need to consider if we want implicit cast operators for JsonNumber, JsonBoolean and JsonString at all.

JsonNumber number = float.PositiveInfinity; // throws

_Originally posted by @ahsonkhan in dotnet/corefx#39917 (comment)

Due to lack of recent activity, this issue has been marked as a candidate for backlog cleanup. It will be closed if no further activity occurs within 14 more days. Any new comment (by anyone, not necessarily the author) will undo this process.

This process is part of the experimental issue cleanup initiative we are currently trialing in a limited number of areas. Please share any feedback you might have in the linked issue.

This issue will now be closed since it had been marked no recent activity but received no further activity in the past 14 days. It is still possible to reopen or comment on the issue, but please note that the issue will be locked if it remains inactive for another 30 days.