fix!: pin corepack version

BREAKING CHANGE: This package now requires a minimum Node.js of v16.20.0.

`corepack` v0.17.0 which ships with Node.js v16 seems completely broken (at least on my machine!) so we can't rely on the version shipped with Node.js.  This adds `corepack` as a dev dependency, and since the minimum version is Node.js v16.20.0, we have to bump our minimum version as well.

This also simplifies finding the `corepack` executable.

README.md

@@ -168,7 +168,7 @@ It's recommended to install `midnight-smoker` as a dev dependency, if you want t
 
 Run `npm install midnight-smoker --save-dev`.
 
-- Node.js versions supported: `^16.13.0 || ^18.0.0 || ^20.0.0`
+- Node.js versions supported: `^16.20.0 || ^18.0.0 || ^20.0.0`
 - Minimum `npm` version supported: `v7.0.0`
 
 While odd-numbered Node.js releases _may_ work, they are not tested on and not officially supported.

package.json

@@ -83,6 +83,7 @@
   "dependencies": {
     "@types/semver": "7.5.0",
     "chalk": "4.1.2",
+    "corepack": "0.19.0",
     "debug": "4.3.4",
     "execa": "5.1.1",
     "lilconfig": "2.1.0",
@@ -133,7 +134,7 @@
     "unexpected-sinon": "11.1.0"
   },
   "engines": {
-    "node": "^16.13.0 || ^18.0.0 || ^20.0.0",
+    "node": "^16.20.0 || ^18.0.0 || ^20.0.0",
     "npm": ">=7"
   },
   "overrides": {