feat: mark Entity and Model to be regular classes

The Model/Entity being abstract caused compilation errors as TS 4.2
reinforces abstract checks.

https://devblogs.microsoft.com/typescript/announcing-typescript-4-2/
microsoft/TypeScript#36392

Signed-off-by: Raymond Feng <enjoyjava@gmail.com>

packages/repository/src/model.ts

@@ -309,7 +309,7 @@ function asObject(value: any, options?: Options): any {
 /**
  * Base class for models
  */
-export abstract class Model {
+export class Model {
   static get modelName(): string {
     return this.definition?.name || this.name;
   }
@@ -418,7 +418,7 @@ export abstract class ValueObject extends Model implements Persistable {}
 /**
  * Base class for entities which have unique ids
  */
-export abstract class Entity extends Model implements Persistable {
+export class Entity extends Model implements Persistable {
   /**
    * Get the names of identity properties (primary keys).
    */