Add build.ps1 for multi-Roslyn build orchestration

[Copilot]

The project packages both Roslyn 3.10 and 4.4 analyzer variants in a single NuGet package. Running dotnet pack directly fails because the Roslyn 4.4 variant uses alternate output paths and must be built explicitly before packing.

Changes

New build.ps1

• Three parameter sets: -Build (default), -Test, -Pack
• Builds baseline (Roslyn 3.10) for all TFMs, then Roslyn 4.4 variant for netstandard2.0
• Supports -VersionSuffix and -PackageReleaseNotesFile for pack operations
• Strict error handling with fail-fast on non-zero exit codes

CI Workflow

• Build step: dotnet pwsh ./build.ps1 -Build
• Test step: dotnet test --no-build (uses direct command to avoid exit code masking)
• Pack step: & ./build.ps1 -Pack (already in PowerShell shell context)

Integration Tests

• Updated to use & ../../build.ps1 -Pack instead of dotnet pack

Documentation

• Added "Building from Source" section to README.md

Usage

# Build both Roslyn variants
dotnet pwsh ./build.ps1

# Run tests (note: use dotnet test directly for CI)
dotnet test --no-build --configuration Release

# Create package
dotnet pwsh ./build.ps1 -Pack -VersionSuffix "beta1"

The script is directory-independent (Push/Pop-Location) and properly fails on any errors without masking test failures.

Original prompt

Implement what's described in issue #208.

Do not ask for a review until complete. Test your changes to make sure everything is working as expected.

The full plan, should you need to consult it can be found in plan.md in the root of the repo.

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