housekeeping: webpack@4 + babel@7 (via #2020)

* babel@7

* structure styles within LESS entrypoint

...instead of importing them among JS modules

* use ESM syntax for entrypoint exports

...because webpack@4 doesn't tolerate import syntax + CJS export syntax

* use property access instead of destructuring `process.env` usage

...because webpack@4 doesn't rewrite destructured access

* webpack@4

* base config builder

* asset build configurations

* dev server configuration

* size audit configuration

* remove legacy validation worker assets

* skip needless minification in `deps-size`

* update package scripts

* remove legacy configs

* prettier

* remove unused Babel plugins

* use correct jsx loader in Mocha

* update eslint

* eslint fixes

.babelrc

@@ -1,29 +1,49 @@
 {
   "presets": [
-    "es2015",
-    "react",
-    "stage-0"
+    [
+      "@babel/env",
+      {
+        "targets": {
+          "browsers": [
+            /* benefit of C/S/FF/Edge only? */
+            "> 1%",
+            "last 2 versions",
+            "Firefox ESR",
+            "not dead",
+          ]
+        },
+        "useBuiltIns": "entry",
+        "corejs": "2"
+      }
+    ],
+    "@babel/preset-react"
   ],
   "plugins": [
-  "transform-runtime",
-  "transform-async-to-generator",
-  "transform-es2015-constants",
-  [
-    "module-alias",
     [
+      "@babel/plugin-transform-runtime",
       {
-        "expose": "plugins",
-        "src": "src/plugins"
-      },
+        "corejs": "2"
+      }
+    ],
+    "@babel/plugin-proposal-class-properties",
+    "@babel/plugin-proposal-optional-chaining",
+    [
+      "transform-react-remove-prop-types",
       {
-        "expose": "test",
-        "src": "test"
-      },
+        "additionalLibraries": [
+          "react-immutable-proptypes"
+        ]
+      }
+    ],
+    [
+      "babel-plugin-module-resolver",
       {
-        "expose": "src",
-        "src": "src"
+        "alias": {
+          "plugins": "./src/plugins",
+          "test": "./test",
+          "src": "./src"
+        }
       }
     ]
   ]
-]
 }

.eslintrc

@@ -15,10 +15,7 @@
 
   "extends": ["eslint:recommended", "plugin:react/recommended"],
 
-  "plugins": [
-    "react",
-    "import"
-  ],
+  "plugins": ["react", "import"],
 
   "rules": {
     "semi": [2, "never"],
@@ -27,13 +24,14 @@
     "no-unused-vars": 2,
     "no-multi-spaces": 1,
     "camelcase": 1,
-    "no-use-before-define": [2,"nofunc"],
+    "no-use-before-define": [2, "nofunc"],
     "no-underscore-dangle": 0,
     "no-unused-expressions": 1,
     "comma-dangle": 0,
-    "no-console": ["error", { allow: ["warn", "error"] }],
+    "no-console": ["error", { "allow": ["warn", "error"] }],
     "react/jsx-no-bind": [1, { "allowArrowFunctions": true }], // TODO: make this an error
     "react/display-name": 0,
-    "import/no-extraneous-dependencies": [2]
+    "import/no-extraneous-dependencies": [2],
+    "no-useless-escape": 1
   }
 }

.prettierrc.yaml

@@ -0,0 +1,5 @@
+semi: false
+trailingComma: es5
+endOfLine: lf
+requirePragma: true
+insertPragma: true