fix: fix passing a single Node to mermaid.init()

Passing a single Node to mermaid.init() results in an error, as it
calls `new NodeList()`, which causes `TypeError: Illegal constructor`.

See 5597cf45bf/src/mermaid.ts (L73)

If we instead use the `ArrayLike` interface, we can just use a simple
array, instead of a NodeList.

I've also added a basic test case, by mocking the `mermaidAPI.render()`
function so it isn't called, as the d3 functions don't work in Node.JS.
The mocks are a bit messy, since
 a) Jest doesn't fully support ESM yet, and
 b) mermaidAPI is frozen with `Object.freeze()`,
but the mermaidAPI mocks work as long as we keep them simple.

Fixes: c68ec54fdd

src/__mocks__/mermaidAPI.ts

@@ -0,0 +1,47 @@
+/**
+ * Mocks for `./mermaidAPI`.
+ * 
+ * We can't easily use `jest.spyOn(mermaidAPI, "function")` since the object
+ * is frozen with `Object.freeze()`.
+ */
+import * as configApi from '../config';
+
+import { addDiagrams } from '../diagram-api/diagram-orchestration';
+import Diagram from '../Diagram';
+
+// Normally, we could just do the following to get the original `parse()`
+// implementation, however, requireActual isn't currently supported in Jest
+// for ESM, see https://github.com/facebook/jest/issues/9430
+// and https://github.com/facebook/jest/pull/10976
+// const {parse} = jest.requireActual("./mermaidAPI");
+
+let hasLoadedDiagrams = false;
+function parse(text: string, parseError?: Function): boolean {
+  if (!hasLoadedDiagrams) {
+    addDiagrams();
+    hasLoadedDiagrams = true;
+  }
+  const diagram = new Diagram(text, parseError);
+  return diagram.parse(text, parseError);
+}
+
+// original version cannot be modified since it was frozen with `Object.freeze()`
+export const mermaidAPI = {
+  render: jest.fn(),
+  parse,
+  parseDirective: jest.fn(),
+  initialize: jest.fn(),
+  getConfig: configApi.getConfig,
+  setConfig: configApi.setConfig,
+  getSiteConfig: configApi.getSiteConfig,
+  updateSiteConfig: configApi.updateSiteConfig,
+  reset: () => {
+    configApi.reset();
+  },
+  globalReset: () => {
+    configApi.reset(configApi.defaultConfig);
+  },
+  defaultConfig: configApi.defaultConfig,
+}
+
+export default mermaidAPI;