Add net10 ImmutableDictionary.CreateRangeWithOverwrite

[SimonCropp]

Two overloads, with and without a key comparer, both net10. Scanning the whole System.Collections.Immutable namespace confirmed these are the only WithOverwrite members anywhere in it, so this is the complete addition rather than one corner of a family.

Scope: netcoreapp2.1 through net9.0

ImmutableDictionary is in the shared framework from netcoreapp2.1; below that it is a separate NuGet package. Extending it on net461, net47x, net48, netstandard2.0 or netstandard2.1 would mean taking a dependency Polyfill does not have today and introducing a new feature constant — plus the Polyfill.targets, Identifiers.sharedIdentifiers and per-framework-count wiring that goes with one — for the sake of two members. The guard is NETCOREAPP2_1_OR_GREATER && !NET10_0_OR_GREATER, and both members carry a //Note: saying so, since #### ImmutableDictionary gives no hint of a floor.

Implemented over a builder, and checked against the BCL rather than assumed

The natural implementation is a builder with indexer assignment, since the indexer already has last-wins semantics. Verified on net11 that it is indistinguishable from the real thing:

• later entries overwrite earlier ones — a=1, b=2, a=3, c=4, b=5 yields three entries with a=3 and b=5
• an empty range returns the shared Empty instance, by reference, not a fresh dictionary
• a supplied comparer is honoured and preserved by reference, including for an empty range
• a null comparer means EqualityComparer<TKey>.Default

There was exactly one difference, and it is the kind that is invisible until someone catches the exception: for a null key the builder indexer reports paramName as key, where the BCL overload reports the KeyValuePair member as Key. The key is now checked before the assignment so the two match, and there is a test pinning it.

Why the API exists

Worth stating, because it is the whole point: ImmutableDictionary.CreateRange throws ArgumentException on the same input that CreateRangeWithOverwrite accepts. One of the tests asserts both halves of that, so the distinction is not just described in a doc comment.

Verification

Solution clean in Release, Consume clean across all 22 TFMs, tests green on net11.0 (1755), net10.0 (1755), net9.0 (1755), net8.0 (1752), net462 (1695), plus PublicTests, EmbeddedTests, UnsafeTests, NoRefsTests and NoExtrasTests. The six new tests were also run on netcoreapp3.1 via roll-forward, since that is the bottom of the polyfill's window and its runtime is not installed here.

API count 1170 → 1172.