Add item-based AggregateConfigInput support, XML handler, and .NET 10

[richardsondev]

Summary

Adds an item-based way to use the package, modelled on Azure.Bicep.MSBuild: declare <AggregateConfigInput> items and the package's own AggregateConfigCompile target aggregates them during the build, with no custom target required. The legacy direct task invocation keeps working unchanged.

Alongside that, this PR adds an XML file handler, moves file writes to async, and brings the repository to the .NET 10 SDK.

Item-based configuration

<ItemGroup>
  <AggregateConfigInput Include="configs\*.yml"
                        OutputFile="$(IntermediateOutputPath)aggregated\config.json"
                        OutputType="Json"
                        AddSourceProperty="true" />
  <EmbeddedResource Include="$(IntermediateOutputPath)aggregated\config.json" />
</ItemGroup>

• build/ and buildMultiTargeting/ props and targets are shipped in the package. The props file registers the task and the AggregateConfigInput build action.
• AggregateConfigCompile runs before PrepareForBuild by default so generated files exist before resources are embedded. AggregateConfigCompileBeforeTargets, AggregateConfigCompileAfterTargets and AggregateConfigCompileDependsOn control the timing.
• The target batches on OutputFile and uses Inputs/Outputs for incremental builds. Missing OutputType defaults to Json; missing OutputFile defaults to $(AggregateConfigOutputPath)%(Filename).json.
• Multi-targeting projects run the target once per target framework; the outer build is skipped.

XML support

• FileType.Xml reads and writes .xml. Root children become properties, attributes become @name, text next to attributes becomes #text, and repeated elements become arrays. JSON arrays are written as a wrapper element with one child per item.
• Output uses a UTF-8 declaration and platform newlines.
• FileType.Yml is added with Yaml kept as an alias.

Other changes

• IFileSystem.WriteAllTextAsync replaces the synchronous write and the unused read helpers; every handler returns a Task from WriteOutput.
• Tests and CI target the .NET 10 SDK. The build and integration workflows install the 8.0, 9.0 and 10.0 SDKs so the multi-targeting test project builds.
• New PackageStructureTests build a single-target and a multi-target project against the freshly packed package, verifying both package layouts, item-based aggregation and the legacy invocation. The integration project also embeds three item-based outputs and checks them.
• The format round-trip unit test now actually runs: its data source previously produced no cases because enum aliases made every permutation impossible.
• README and a new migration guide document the item-based approach, metadata defaults and the XML mapping. An example project with sample configs lives in examples/.

Notes for reviewers

• The InputDirectory semantics are unchanged: for item-based inputs the task merges every file of InputType in the directory that contains the items. Items sharing an OutputFile should live in the same directory.
• PackageStructureTests need the freshly packed package. Locally: dotnet build src/Task -c Release -p:Version=0.0.1-local, then run dotnet test test/dirs.proj -c Release -p:Version=0.0.1-local -p:UseLocalPackageVersion=true with RestoreAdditionalProjectSources pointing at src/Task/bin/Release.
• Moving the test projects to net10.0 should also unblock the Dependabot bump in Bump the nuget-dependencies group with 18 updates #98, which fails because Microsoft.Build 18.x no longer supports net9.0 test hosts.

Relates to #17 (sample projects), #21 (incremental output for the item-based path) and #33 (XML is a first step toward app.config support).

Test plan

• dotnet test src/dirs.proj -c Release -warnaserror: 70 unit tests pass
• dotnet test test/dirs.proj -c Release -warnaserror -p:UseLocalPackageVersion=true against a freshly packed package: 15 integration tests pass, including the single-target and multi-target package builds
• CI green on Windows, Linux and macOS