ts2fable generates invalid DU case names

[baronfel]

for this TS definition (specifically the character union for the separator field):

	export interface ResourceLabelFormatting {
		label: string; // myLabel:/${path}
		// For historic reasons we use an or string here. Once we finalize this API we should start using enums instead and adopt it in extensions.
		// eslint-disable-next-line vscode-dts-literal-or-types
		separator: '/' | '\\' | '';
		tildify?: boolean;
		normalizeDriveLetter?: boolean;
		workspaceSuffix?: string;
		workspaceTooltip?: string;
		authorityPrefix?: string;
		stripPathStartingSeparator?: boolean;
	}

ts2fable 0.8.0-build.616 generates the following types:

    type [<StringEnum>] [<RequireQualifiedAccess>] ResourceLabelFormattingSeparator =
        | [<CompiledName "/">] ``/``
        | [<CompiledName "\\">] ``\\``
        | [<CompiledName "">] Empty

These DU case names are invalid and as a result throw errors 883 (Invalid namespace, module, type or union case name) and 53 (Discriminated union cases and exception labels must be uppercase identifiers)