Support charmcraft.yaml as single configuration file

[babakks]

The latest charm development style is to use a single charmcraft.yaml file instead of multiple individual files (e.g., metadata.yaml, config.yaml, or actions.yaml).

The extension should support this style while keeping the current behavior for backward-compatibility. This also affects the way we show config.yaml or actions.yaml in the sidebar tree. We need to keep things separated in the tree (i.e., not nesting individual actions/configs under the charmcraft.yaml node), but we need to change the title of the node to Actions or Configs when they're defined in the charmcraft.yaml file.

Also, a warning diagnostic message should be displayed on YAML files to hint users to use a single charmcraft.yaml file. In addition to that, if a node is defined in more than one YAML file, a diagnostics message should complain about it.

[Thanhphan1147]

+1. Looking forward to this feature!