refactor: katex.d.ts -> typings-external.ts

This way the .d.ts will not be deleted when filse are deleted through
rimraf

src/augmentation.ts

@@ -1,4 +1,4 @@
-import type { KatexOptions } from './katex';
+import type { KatexOptions } from './typings-external';
 
 declare module '@ckeditor/ckeditor5-core' {
 	interface EditorConfig {

src/typings-external.ts

@@ -1,14 +1,17 @@
-interface MathJax3 {
+/**
+ * Basic typings for third party, external libraries (KaTeX, MathJax).
+ */
+export interface MathJax3 {
 	version: string;
 	tex2chtmlPromise?: ( input: string, options: { display: boolean } ) => Promise<HTMLElement>;
 	tex2svgPromise?: ( input: string, options: { display: boolean } ) => Promise<HTMLElement>;
 }
 
-interface MathJax2 {
+export interface MathJax2 {
 	Hub: { Queue: ( callback: [string, MathJax2['Hub'], string | HTMLElement] | ( () => void ) ) => void };
 }
 
-interface Katex {
+export interface Katex {
 	render( equation: string, el: HTMLElement, options: KatexOptions ): void;
 }
 

src/ui/mainformview.ts

@@ -16,7 +16,7 @@ import { Locale, FocusTracker, KeystrokeHandler } from 'ckeditor5/src/utils';
 import { extractDelimiters, hasDelimiters } from '../utils';
 import MathView from './mathview';
 import '../../theme/mathform.css';
-import type { KatexOptions } from '../katex';
+import type { KatexOptions } from '../typings-external';
 
 const { check: checkIcon, cancel: cancelIcon } = icons;
 

src/ui/mathview.ts

@@ -1,7 +1,7 @@
 import { View } from 'ckeditor5/src/ui';
+import type { KatexOptions } from '../typings-external';
 import { renderEquation } from '../utils';
 import type { Locale } from 'ckeditor5/src/utils';
-import type { KatexOptions } from '../katex';
 
 export default class MathView extends View {
 	public value: string;

src/utils.ts

@@ -5,7 +5,7 @@ import type {
 } from 'ckeditor5/src/engine';
 import { BalloonPanelView } from 'ckeditor5/src/ui';
 import { CKEditorError, type PositioningFunction } from 'ckeditor5/src/utils';
-import type { KatexOptions, MathJax2, MathJax3 } from './katex';
+import type { KatexOptions, MathJax2, MathJax3 } from './typings-external';
 
 export function getSelectedMathModelWidget(
 	selection: DocumentSelection