fix: remove sequelize type dependency completely

ts-ignore comments are stripped from declaration files: microsoft/TypeScript#38628

It'd be possible to use some kind of codegen to add them back in, but it's easier and more correct to define what we need explicitly from the sequelize type definitions.

src/storage/sequelize.ts

@@ -1,12 +1,34 @@
 import { UmzugStorage } from './contract';
 import { SetRequired } from 'type-fest';
-// eslint-disable-next-line @typescript-eslint/prefer-ts-expect-error
-// @ts-ignore (Avoid type errors for non-sequelize users. Can't use ts-expect-error; this _won't_ be an error when sequelize is installed)
-import type { Sequelize as SequelizeType, Model as ModelClass } from 'sequelize';
 
 interface ModelTempInterface extends ModelClass, Record<string, any> {}
 
-type ModelClassType = typeof ModelClass & (new (values?: object, options?: any) => ModelTempInterface);
+/**
+ * Minimal structure of a sequelize model, defined here to avoid a hard dependency.
+ * The type expected is `import { Model } from 'sequelize'`
+ */
+export interface ModelClass {
+	tableName: string;
+	sequelize?: SequelizeType;
+	getTableName(): string;
+	sync(): Promise<void>;
+	findAll(options?: {}): Promise<any[]>;
+	create(options: {}): Promise<void>;
+	destroy(options: {}): Promise<void>;
+}
+
+/**
+ * Minimal structure of a sequelize model, defined here to avoid a hard dependency.
+ * The type expected is `import { Sequelize } from 'sequelize'`
+ */
+export interface SequelizeType {
+	getQueryInterface(): any;
+	isDefined(modelName: string): boolean;
+	model(modelName: string): any;
+	define(modelName: string, columns: {}, options: {}): {};
+}
+
+type ModelClassType = ModelClass & (new (values?: object, options?: any) => ModelTempInterface);
 
 interface _SequelizeStorageConstructorOptions {
 	/**