Allow use of default slice when defining an Instance

Originally reported at https://github.com/HL7/fhir-shorthand/issues/183.

SUSHI should allow authors to refer to the default slice when defining an Instance, even if the default slice is not defined. This allows authors to create elements that do not have a slice name on an element with open slicing without generating the "Use slice names in rule paths when possible" warning.

No file or test is named. Start by tracing how SUSHI handles Instance definitions, slice names, and the "Use slice names in rule paths when possible" warning; verify the behavior with an open-sliced element whose default slice is not defined. Done means the unnamed default slice can be referenced without that warning.

Written by the indexing model from the issue text.