Fix Nickel quoted_enum_tag formatting by considering it a leaf

[ErinvanderVeen]

Mentioned in #547 but doesn't resolve all of it.

From #547:

When parsing a quoted_enum_tag, tree-sitter-nickel defers to its scanner. The scanner produces a quoted_enum_tag_start, which is taken by Topiary and produced correctly. The parser then produces a chunk_literal_single containing only the contents of the following string. I.e. the last quote ending the string is completely dropped by tree-sitter-nickel.