fix(typescript-fetch): use type modifier on imports for discriminator.mappedModels (#21397)

* typescript-fetch: use `type` modifier on imports for `discriminator.mappedModels`

This ensures that the generated code works when `--verbatimModuleSyntax` is enabled for the TypeScript compiler.
Regular imports already use the `type` modifier, so this should not be a breaking change.

* update samples

Author: Noah1989

modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache

@@ -13,7 +13,7 @@ import {
 {{/hasImports}}
 {{#discriminator}}
 {{#discriminator.mappedModels}}
-import { {{modelName}}, {{modelName}}FromJSONTyped, {{modelName}}ToJSON, {{modelName}}ToJSONTyped } from './{{modelName}}{{importFileExtension}}';
+import { type {{modelName}}, {{modelName}}FromJSONTyped, {{modelName}}ToJSON, {{modelName}}ToJSONTyped } from './{{modelName}}{{importFileExtension}}';
 {{/discriminator.mappedModels}}
 {{/discriminator}}
 {{>modelGenericInterfaces}}

samples/client/others/typescript-fetch/self-import-issue/models/AbstractUserDto.ts

@@ -21,8 +21,8 @@ import {
     BranchDtoToJSONTyped,
 } from './BranchDto';
 
-import { InternalAuthenticatedUserDto, InternalAuthenticatedUserDtoFromJSONTyped, InternalAuthenticatedUserDtoToJSON, InternalAuthenticatedUserDtoToJSONTyped } from './InternalAuthenticatedUserDto';
-import { RemoteAuthenticatedUserDto, RemoteAuthenticatedUserDtoFromJSONTyped, RemoteAuthenticatedUserDtoToJSON, RemoteAuthenticatedUserDtoToJSONTyped } from './RemoteAuthenticatedUserDto';
+import { type InternalAuthenticatedUserDto, InternalAuthenticatedUserDtoFromJSONTyped, InternalAuthenticatedUserDtoToJSON, InternalAuthenticatedUserDtoToJSONTyped } from './InternalAuthenticatedUserDto';
+import { type RemoteAuthenticatedUserDto, RemoteAuthenticatedUserDtoFromJSONTyped, RemoteAuthenticatedUserDtoToJSON, RemoteAuthenticatedUserDtoToJSONTyped } from './RemoteAuthenticatedUserDto';
 /**
  * 
  * @export

samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Animal.ts

@@ -13,8 +13,8 @@
  */
 
 import { mapValues } from '../runtime';
-import { Cat, CatFromJSONTyped, CatToJSON, CatToJSONTyped } from './Cat';
-import { Dog, DogFromJSONTyped, DogToJSON, DogToJSONTyped } from './Dog';
+import { type Cat, CatFromJSONTyped, CatToJSON, CatToJSONTyped } from './Cat';
+import { type Dog, DogFromJSONTyped, DogToJSON, DogToJSONTyped } from './Dog';
 /**
  * 
  * @export

samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ParentWithNullable.ts

@@ -13,7 +13,7 @@
  */
 
 import { mapValues } from '../runtime';
-import { ChildWithNullable, ChildWithNullableFromJSONTyped, ChildWithNullableToJSON, ChildWithNullableToJSONTyped } from './ChildWithNullable';
+import { type ChildWithNullable, ChildWithNullableFromJSONTyped, ChildWithNullableToJSON, ChildWithNullableToJSONTyped } from './ChildWithNullable';
 /**
  * 
  * @export

samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Animal.ts

@@ -13,8 +13,8 @@
  */
 
 import { mapValues } from '../runtime';
-import { Cat, CatFromJSONTyped, CatToJSON, CatToJSONTyped } from './Cat';
-import { Dog, DogFromJSONTyped, DogToJSON, DogToJSONTyped } from './Dog';
+import { type Cat, CatFromJSONTyped, CatToJSON, CatToJSONTyped } from './Cat';
+import { type Dog, DogFromJSONTyped, DogToJSON, DogToJSONTyped } from './Dog';
 /**
  * 
  * @export