Fix issues in type definitions

Author: remcohaszing

index.d.ts

@@ -1,12 +1,12 @@
-import { RemarkMermaidOptions } from 'remark-mermaidjs'
+import type { RemarkMermaidOptions } from 'remark-mermaidjs' with { 'resolution-mode': 'import' }
 
 declare namespace gatsbyRemarkMermaid {
-  type Options = RemarkMermaidOptions
+  export { RemarkMermaidOptions as Options }
 }
 
 declare function gatsbyRemarkMermaid(
   gatsbyRemarkOptions: unknown,
-  options: RemarkMermaidOptions
-): Promise<void>
+  options: gatsbyRemarkMermaid.Options
+): Promise<undefined>
 
 export = gatsbyRemarkMermaid