Add user-defined numeral parsing

[ysangkok]

Description of the change
This makes the underlying Json type polymorphic. You can now plug in a type
to use for numbers, and you can then avoid losing precision when handling JSON
with the Number. I am working on a library for infinite precision numbers in
scientific notation, which I can show if there is interest. One could also use decimals.

Existing users remain compatible with this change since Json is still
available as a type synonym for Json Number. You only get new functionality
if you import the new module.

Note that this feature depends on a new JavaScript runtime:

• https://caniuse.com/wf-json-raw

In the new API, stringify can fail, which happens for example when you emit
invalid JSON or run the code on an old JavaScript runtime. I have tested
manually that it fails gracefully on Node.js v20. But this test isn't included
in the automated test suite, since I am not sure if the test suite should be
doing feature probing. It would be hard to know whether the feature is
'supposed' to work.

Pinging @thomashoneyman and @garyb as I was told on Discord that they might be
interested.

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Checklist:

• Added the change to the changelog's "Unreleased" section with a link to this PR and your username
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation in the README and/or documentation directory
• Added a test for the contribution (if applicable)