refactor: move MaybePromise type to src/types.ts

Suggested-by: https://github.com/mermaid-js/mermaid/pull/5974#discussion_r1810562136
Co-authored-by: Sidharth Vinod <sidharthv96@gmail.com>

packages/mermaid/src/rendering-util/rendering-elements/shapes.ts

@@ -1,5 +1,5 @@
 import type { Entries } from 'type-fest';
-import type { D3Selection } from '../../types.js';
+import type { D3Selection, MaybePromise } from '../../types.js';
 import type { Node, ShapeRenderOptions } from '../types.js';
 import { anchor } from './shapes/anchor.js';
 import { bowTieRect } from './shapes/bowTieRect.js';
@@ -58,7 +58,6 @@ import { waveEdgedRectangle } from './shapes/waveEdgedRectangle.js';
 import { waveRectangle } from './shapes/waveRectangle.js';
 import { windowPane } from './shapes/windowPane.js';
 
-type MaybePromise<T> = T | Promise<T>;
 type ShapeHandler = <T extends SVGGraphicsElement>(
   parent: D3Selection<T>,
   node: Node,

packages/mermaid/src/types.ts

@@ -94,3 +94,10 @@ export interface RenderResult {
    */
   bindFunctions?: (element: Element) => void;
 }
+
+/**
+ * Can be converted back to `T` by awaiting/`Awaited<T>`.
+ *
+ * This is useful for function types that may be either synchronous or asynchronous.
+ */
+export type MaybePromise<T> = T | Promise<T>;