chore(core): temporarily add inquirer as dev dependency

This is only required to make depcheck pass - we're only importing the types,
which are given by `@types/inquirer`, but depcheck doesn't understand this.

I created a PR[1] against depcheck to fix this, but it is hard to know when/if
this patch will land, so this seems like a reasonable workaround given it is
only a development dependency.

[1]: depcheck/depcheck#670

packages/@sanity/core/package.json

@@ -73,7 +73,8 @@
     "@types/fs-extra": "^7.0.0",
     "@types/inquirer": "^6.0.2",
     "@types/resolve-from": "^4.0.0",
-    "@types/semver": "^6.2.3"
+    "@types/semver": "^6.2.3",
+    "inquirer": "^6.0.0"
   },
   "repository": {
     "type": "git",