build(eslint): add tsconfig for all linted files

packages/mermaid-example-diagram/tsconfig.eslint.json

@@ -0,0 +1,11 @@
+{
+  "$schema": "https://json.schemastore.org/tsconfig",
+  "extends": ["./tsconfig.json"],
+  "compilerOptions": {
+    "noEmit": true
+  },
+  "include": [
+    "./src/**/*.spec.js",
+    "./src/**/*.spec.ts" // test files
+  ]
+}