feat(csharp): option to suppress DateOnly/TimeOnly converters (System.Text.Json)

[schani]

Supersedes #2842, fixes #2629.

This is a reimplementation of @findyoucef's PR #2842, which could not be rebased (conflicts with the C# 8 default work plus unrelated dependency churn). The core idea — making the System.Text.Json DateOnly/TimeOnly converter emission optional — is theirs; their authorship is preserved via a Co-authored-by trailer on the commit.

Problem

The System.Text.Json C# renderer unconditionally emits DateOnlyConverter and TimeOnlyConverter helper classes and registers them in Converter.Settings. DateOnly/TimeOnly only exist on .NET 6+, so the generated code does not compile on .NET Standard or older target frameworks (#2629).

Change

A new System.Text.Json-only boolean option:

--[no-]dateonly-timeonly-converters    Emit DateOnly/TimeOnly converters (requires .NET 6 or later) (on by default)

With --no-dateonly-timeonly-converters, both converter classes and their Converters = { ... } registrations are omitted. Nothing else in the generated code references them, so the output still compiles and round-trips. Default output is byte-for-byte identical to master.

The option lives in systemTextJsonCSharpOptions rather than the shared cSharpOptions, so only the System.Text.Json renderer's typed options include it — it is not a silent no-op field on the Newtonsoft options object (the original PR put its flags in the shared options). CSharpTargetLanguage.getOptions() now returns the System.Text.Json superset so the CLI accepts and documents the flag.

Kept from #2842 vs. dropped

• Kept: conditional emission and registration of the two converters, gated on a renderer option — as one flag instead of the original two (no-dateonly/no-timeonly), since DateOnly and TimeOnly were introduced together in .NET 6 and no target wants only one of them.
• Dropped: the csTypeLocal DateOnly→DateTime / TimeOnly→TimeSpan remapping helper (dead code — the C# type mapper never produces DateOnly/TimeOnly property types; dates map to DateTimeOffset); the package.json/package-lock.json dependency bumps; .vscode/tasks.json; and the whitespace reformat of the converter boilerplate (keeping default output byte-identical).

Testing

• Fixture: csharp-SystemTextJson gains a pinned quick-test — unions.json with dateonly-timeonly-converters: false — proving that generated code with the converters suppressed still compiles and round-trips.
• Unit test test/unit/csharp-dateonly-timeonly-converters.test.ts: default output contains both converter classes and registrations; with the flag set it contains neither, while IsoDateTimeOffsetConverter is unaffected.
• Locally green: FIXTURE=csharp-SystemTextJson (236/236 samples), FIXTURE=schema-csharp-SystemTextJson, FIXTURE=csharp, FIXTURE=schema-csharp, and npm run test:unit (165 tests, type tests included).

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