chore: update is-expression and jest (#3253)

This means we pull in the current latest version of acorn, resulting in a single version everywhere, rather than multiple copies of acorn in this repo.

Author: ForbesLindesay

package.json

@@ -6,7 +6,7 @@
   ],
   "devDependencies": {
     "coveralls": "3.0.9",
-    "jest": "24.9.0",
+    "jest": "^26.0.1",
     "prettier": "1.19.1"
   },
   "repository": {

packages/pug-lexer/package.json

@@ -7,7 +7,7 @@
   ],
   "dependencies": {
     "character-parser": "^2.2.0",
-    "is-expression": "^3.0.0",
+    "is-expression": "^4.0.0",
     "pug-error": "^1.3.3"
   },
   "devDependencies": {