fix: upgrade cross-spawn and strip-ansi (node 8+)

BREAKING CHANGE: these deps now require node 8 or higher

Node 8 went out of support in Dec 2019, but is still popular enough that most
libraries kinda support it (including me).

package.json

@@ -32,18 +32,18 @@
     "printWidth": 100
   },
   "dependencies": {
-    "cross-spawn": "^5.1.0",
+    "cross-spawn": "^7.0.3",
     "node-cleanup": "^2.1.2",
     "ps-tree": "^1.2.0",
     "string-argv": "^0.1.1",
-    "strip-ansi": "^4.0.0"
+    "strip-ansi": "^6.0.0"
   },
   "peerDependencies": {
     "typescript": "*"
   },
   "devDependencies": {
     "chai": "^4.2.0",
-    "fs-extra": "^5.0.0",
+    "fs-extra": "^8.1.0",
     "mocha": "^7.1.2",
     "mocha-eventually": "^1.1.0",
     "sinon": "^9.0.2",