feat: add mermaidAPI.registerDiagram()

Exposes the registerDiagram() function publically as
`mermaid.mermaidAPI.registerDiagram` so that users can add their
own diagrams at bundle-time.

This is instead of using the lazyLoadedDiagrams config setting.

cypress/integration/other/external-diagrams.spec.js

@@ -0,0 +1,13 @@
+describe('mermaid', () => {
+  describe('registerDiagram', () => {
+    it('should work on @mermaid-js/mermaid-mindmap and mermaid-example-diagram', () => {
+      const url = 'http://localhost:9000/external-diagrams-mindmap.html';
+      cy.visit(url);
+
+      cy.get('svg', {
+        // may be a bit slower than normal, since vite might need to re-compile mermaid/mermaid-mindmap/mermaid-example-diagram
+        timeout: 10000,
+      }).matchImageSnapshot();
+    });
+  });
+});