Seems like VS Code may have started using lowercase extension folders recently (our identifier is `Dart-Code.flutter` but now the folder is named `dart-code.flutter`), so this makes the check not sensitive to casing.

Also reuse extension identifier for download link

Fixes #15198.

* Show 32/64bit in VS Code headings on Windows

Fixes #14949.

Looks in default install locations on Mac, Linux and Windows for VS Code. If found, looks in default extension location to see if Dart Code is installed.

If VS Code is not installed, nothing is reported. If VS Code is installed without Dart Code, a warning is shown.