build: change package export to mermaid.core.mjs

`mermaid.core.mjs` should be the default export.
This is because it does not bundle `node_modules/`, allowing users of
mermaid to bundle dependencies themselves,
using Webpack/ESBuild/others.

cypress/platform/bundle-test.js

@@ -1,4 +1,4 @@
-import mermaid from '../../dist/mermaid';
+import mermaid from '../../dist/mermaid.core.mjs';
 
 let code = `flowchart LR
 Power_Supply --> Transmitter_A

package.json

@@ -2,13 +2,13 @@
   "name": "mermaid",
   "version": "9.1.6",
   "description": "Markdownish syntax for generating flowcharts, sequence diagrams, class diagrams, gantt charts and git graphs.",
-  "main": "dist/mermaid.min.js",
-  "module": "dist/mermaid.esm.min.mjs",
+  "main": "dist/mermaid.core.mjs",
+  "module": "dist/mermaid.core.mjs",
   "types": "dist/mermaid.d.ts",
   "exports": {
     ".": {
       "require": "./dist/mermaid.min.js",
-      "import": "./dist/mermaid.esm.min.mjs",
+      "import": "./dist/mermaid.core.mjs",
       "types": "./dist/mermaid.d.ts"
     },
     "./*": "./*"