Determine if current project is a Quarkus project before adding Quarkus extensions

[xorye]

The extension currently lets the user try to add Quarkus extensions even if a Maven Quarkus project is not open.

It would be great if we had something like this:

1. Open a folder that does not contain a Quarkus project
2. Invoke the "Quarkus: Add extensions to current project" command (which starts the vscode-quarkus extension since it is one of the activation events)
3. vscode-quarkus then detects that we are not in a Quarkus project, and displays an error message.

[xorye]

After calling "Quarkus: Add extensions to current project" command, there is an error message if a pom.xml cannot be found, but it would be great if we took it one step further and ensured that if we did find a pom.xml, ensure that it is for a Quarkus project.