`builtins.fromJSON` with true lossless JSON number support

[roberth]

Is your feature request related to a problem?

The original JSON specification only specifies the syntax of numbers, and nothing about lossy representations of them. Converting JSON numbers to floats is lossy.

Furthermore, floats are undesirable due to their complexity and occasional implementation differences, especially in "corner cases" like NaN semantics.
Inheriting this complexity is not appropriate for a language with a focus on reproducibility, so we should make this complexity possible to avoid.

Proposed solution

As of writing, we only have a single fromJSON function without extra parameters.
Add a fromJSONWith params str primop where params.losslessNumbers = true; is the new default behavior.
When true it parses the JSON without converting numbers to floats. Instead, it converts them to a wrapper object of the shape { _type = "number"; number = "6.2"; }, where the attribute value of number corresponds to the unaltered JSON number string as parsed.

Have a corresponding toJSONWith that can unpack these number wrappers.

Alternative solutions

It's tempting to lowercase the E, and/or drop the +, as these do not normally affect the meaning of the number, but we can't be sure how an application treats these differences.

Additional context

• Accounting for arbitrary precision numerical literals nlohmann/json#1849
• Topic Improve representation for "floats" and fix repro bugs #8274

Checklist

• checked latest Nix manual (source)
• checked open feature issues and pull requests for possible duplicates

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