Allow opening the `extends.file` attribute of a service

[rcjsuen]

services:
  test2:
    extends:
      service: test
      file: ./compose.other.yaml

We should allow opening compose.other.yaml as a link. We will need to augment the textDocument/documentLink request for this. This should be achievable by modifying the code below to consider an extends object and then by reusing the types.AbsolutePath function. We can also reference what was done in createIncludeLink to create the appropriate link result.

docker-language-server/internal/compose/documentLink.go

Lines 110 to 122 in 351618a

	case "services":
	if mappingNode, ok := n.Value.(*ast.MappingNode); ok {
	for _, node := range mappingNode.Values {
	if serviceAttributes, ok := node.Value.(*ast.MappingNode); ok {
	for _, serviceAttribute := range serviceAttributes.Values {
	link := createImageLinks(serviceAttribute)
	if link != nil {
	links = append(links, *link)
	}
	}
	}
	}
	}

[rcjsuen]

According to @ndeloof, the attribute can also be an oci:// ref or a Git one so we should create a link if it's a local file. We can worry about supporting opening a Git link separately and later.