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feat(json): add free `to_json` function (#3850)

Add a free `@json.to_json(value)` function, symmetric with `@json.from_json`,
delegating to the `ToJson` trait method. This gives a first-class free-function
form for converting any `ToJson` value to `Json`, so callers can prefer it over
the implicitly-promoted `value.to_json()` method.

Prerequisite for the `implicit_impl_as_method` (E0079) migration: once this
lands, the per-package PRs (first one #3849) can deprecate the promoted `to_json`
method in favour of `@json.to_json`.

The doc example is a checked (`mbt check`) doctest and uses `@debug.debug_inspect`
rather than `inspect`, since `Json`'s `Show` impl is deprecated in favour of
`Debug`.

Verified: `moon check --deny-warn --target all` clean; `moon test -p
moonbitlang/core/json --target all` green (185 tests x 4 backends).

Reviewed by Codex CLI: signature mirrors from_json, trait delegation is correct,
the checked doctest is well-formed and exercises @json.to_json, @debug.debug_inspect
correctly avoids the deprecated Json Show (Debug repr matches), and the change is
scoped to json/ with no deprecation added.

Signed-off-by: Codex CLI <codex@openai.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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