build(deps): override yargs to ^18 so c8 runs on current Node

Problem: c8 pulls yargs 17.7.2, whose extensionless ./yargs file crashes under the Node 22.20+/24/26 module-loader changes (require is not defined in ES module scope), reddening the `node` CI leg.

Goal: keep the existing c8 coverage setup working on current Node without pulling in the native-coverage tooling change.

Changes:
- add npm overrides forcing yargs to ^18.0.0 (only c8 consumes it)

Author: ChristianMurphy

package.json

@@ -66,6 +66,9 @@
   ],
   "license": "MIT",
   "name": "mdast-util-from-markdown",
+  "overrides": {
+    "yargs": "^18.0.0"
+  },
   "prettier": {
     "bracketSpacing": false,
     "semi": false,