mutual exclusivity of reference and genome

Signed-off-by: eternal-flame-AD <yume@yumechi.jp>

Author: eternal-flame-AD

js/igv.d.ts

@@ -574,7 +574,7 @@ type HostedGenomes = "hs1" | "chm13v1.1" | "hg38" | "hg38_1kg" | "hg19" |
 
 export type GenomeDef = (HostedGenomes | (string & {})) | ReferenceGenome;
 
-export type GenomeOpt = { reference: GenomeDef } | { genome: GenomeDef };
+export type GenomeOpt = { reference: GenomeDef, genome?: never } | { genome: GenomeDef, reference?: never };
 
 /**
  * To define or customize a reference genome the reference property can be used.