"Select All" functionality for OpenShift Terminal

See [this comment from Victor](https://github.com/redhat-developer/vscode-openshift-tools/pull/3065#issuecomment-1693730583).

There are context menu items for cut, copy and paste, but not "Select All".

I don't know if it's feasible to add another context menu, but it would be nice to have some method of doing "Select All" even if it's not in a context menu.