Log Content Builder command before execution

[marko1olo]

Fixes #39.

The Content Builder templates currently print Content Builder started, but the generated MSBuild target does not show the command line that is about to be executed. This adds a high-importance MSBuild Message immediately before the Exec task in all three generated BuildContent.targets files, using the same $(ContentCommand) $(ContentArgs) expression that the Exec task runs.

Validation:

• Parsed all three changed .targets files as XML.
• git diff --check passed.
• dotnet pack CSharp\MonoGame.Templates.CSharp.csproj -p:Configuration=Release /p:Version=3.8.5 passed.
• Installed the generated nupkg into an isolated custom template hive.
• Generated mgblankmgcbstartkit into C:\tmp\mgtemplates-39-smoke\CommandMessageGame.
• Confirmed generated CommandMessageGame.Content\BuildContent.targets contains the new Message task.
• Restored the generated content project, then built CommandMessageGame.DesktopGL.csproj with DOTNET_ROLL_FORWARD=Major; build succeeded and printed the generated Content Builder command line before [I] Starting Content Builder.

AI-assisted contribution: implementation and verification were performed with OpenAI Codex under my direction.