fix transformPaths plugin type to be compatible with interface CustomTransformers

[ambroseus]

related issue: #11
caused by: jaredpalmer/tsdx#374 (comment)

current transformPaths declaration is not compatible with TS CustomTransformers
it causes type check error when using with rollup-plugin-typescript2 'transformers' option

fix:
-- afterDeclarations(....): ts.Transformer<ts.SourceFile>
++ afterDeclarations(....): ts.Transformer<ts.SourceFile | ts.Bundle>

export function transformPaths(program?: ts.Program, config: Config = {}) {
   ......
    afterDeclarations(
      transformationContext: TransformationContext
    ): ts.Transformer<ts.SourceFile | ts.Bundle> {
      return plugin.before(transformationContext)
    },
  }

  return plugin
}

@zerkalica I will create PR soon :)