Audit TS module/target config and fix extensionless relative imports across packages

## Problem

Several relative imports in `packages/sdk` (and likely elsewhere in the
monorepo) omit the `.js` extension or import a directory instead of its
`index.js` file, e.g. `from '../../entities'` instead of
`from '../../entities/index.js'`.

Since `@editorjs/sdk`'s `package.json` has `"type": "module"`, Node's ESM
resolver is strict about this: a value import through a bare directory
specifier throws `ERR_UNSUPPORTED_DIR_IMPORT`, and a missing extension on a
file specifier throws `ERR_MODULE_NOT_FOUND`. Type-only (`import type`)
specifiers with the same issue don't break at runtime (TS strips them
entirely), but they do break `.d.ts` resolution for consumers using
`moduleResolution: node16`/`nodenext`.

This doesn't surface as a failure anywhere in the repo today, since the only
consumers are Vite (`packages/playground`) and `ts-jest` (test suites) — both
resolve directories/extensions leniently, unlike raw Node ESM. But
`@editorjs/sdk` is published to npm, so any external consumer running it
under plain Node would hit this.

## Known instances

- `packages/sdk/src/entities/InlineTool.ts`
- `packages/sdk/src/entities/BlockTool.ts`
- `packages/sdk/src/entities/BlockTune.ts`
- `packages/sdk/src/entities/EditorjsPlugin.ts`
- `packages/sdk/src/entities/EditorjsAdapterPlugin.ts`
- `packages/sdk/src/entities/BlockToolAdapter.ts`
- `packages/sdk/src/entities/EventBus/events/core/ToolLoadedCoreEvent.ts`
- `packages/sdk/src/entities/EventBus/events/core/SelectionChangedCoreEvent.ts`

Similar patterns likely exist elsewhere in the monorepo, outside `packages/sdk`.

## Proposed scope

- Audit `module`/`moduleResolution`/`target` in each package's tsconfig to
match how it's actually consumed (bundler vs. Node ESM).
- Fix extensionless/directory relative imports repo-wide.
- Consider an ESLint rule (e.g. `import/extensions`) to prevent regressions
going forward.

Bắt đầu với các tệp được liệt kê trong packages/sdk/src/entities và packages/sdk/src/entities/EventBus/events/core, sau đó kiểm tra tsconfig của từng package về các thiết lập module, moduleResolution và target. Kiểm tra các import tương đối để tìm các tệp .js tường minh hoặc các đường dẫn index.js, bao gồm cả các import chỉ dành cho kiểu, và xác minh rằng SDK đã phát hành được resolve trong Node ESM mà không làm hỏng Vite playground hoặc các consumer của ts-jest.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.