refactor: update imports to use ES module syntax and improve test structure (#1350)

- Changed import statements across multiple files to use the .js extension for ES module compatibility.
- Refactored test files to utilize jest's unstable_mockModule for mocking core actions.
- Removed unnecessary spy instances in tests, replacing them with direct mocked function calls.
- Updated TypeScript configuration to target ES2022 and use NodeNext module resolution.
- Removed the tsconfig.spec.json file and adjusted the main tsconfig.json to exclude test files.

__tests__/functions/generate-iissue.ts

@@ -1,4 +1,4 @@
-import {IIssue} from '../../src/interfaces/issue';
+import {IIssue} from '../../src/interfaces/issue.js';
 
 export function generateIIssue(
   partialIssue?: Readonly<Partial<IIssue>>

__tests__/functions/generate-issue.ts

@@ -1,7 +1,7 @@
-import {Issue} from '../../src/classes/issue';
-import {IUserAssignee} from '../../src/interfaces/assignee';
-import {IIssuesProcessorOptions} from '../../src/interfaces/issues-processor-options';
-import {IsoDateString} from '../../src/types/iso-date-string';
+import {Issue} from '../../src/classes/issue.js';
+import {IUserAssignee} from '../../src/interfaces/assignee.js';
+import {IIssuesProcessorOptions} from '../../src/interfaces/issues-processor-options.js';
+import {IsoDateString} from '../../src/types/iso-date-string.js';
 
 export function generateIssue(
   options: IIssuesProcessorOptions,