WT-10967: Add eslint configuration

This adds the lint script to package.yaml, and an .eslintrc.yml with the
configuration for Wt.

For gulpfile.js we have to indicate that it is using the node env, so
eslint doesn't report that require doesn't exist.

Author: RockinRoel

.eslintignore

@@ -0,0 +1,2 @@
+*.min.js
+jquery.js

.eslintrc.yml

@@ -0,0 +1,49 @@
+env:
+  browser: true
+  es2021: true
+extends: eslint:recommended
+overrides: []
+globals:
+  WT_DECLARE_APP_MEMBER: readonly
+  WT_DECLARE_WT_MEMBER: readonly
+  WT_DECLARE_WT_MEMBER_BIG: readonly
+  JavaScriptFunction: readonly
+  JavaScriptConstructor: readonly
+  JavaScriptObject: readonly
+  JavaScriptPrototype: readonly
+parserOptions:
+  ecmaVersion: latest
+rules:
+  # Possible Problems
+  no-constructor-return:
+    - error
+  no-inner-declarations:
+    - off
+  no-unused-vars:
+    - error
+    - argsIgnorePattern: ^_
+      varsIgnorePattern: ^_
+  # Suggestions
+  consistent-return:
+    - error
+  eqeqeq:
+    - error
+  guard-for-in:
+    - error
+  no-implicit-globals:
+    - error
+  no-sequences:
+    - error
+  no-undefined:
+    - error
+  no-var:
+    - error
+  prefer-const:
+    - error
+  # Layout & Formatting
+  # dprint does not reformat comments,
+  # this catches max-len violations in comments:
+  max-len:
+    - error
+    - code: 120
+      tabWidth: 8

src/js/gulpfile.js

@@ -1,3 +1,4 @@
+/* eslint-env node */
 const gulp = require("gulp");
 const rename = require("gulp-rename");
 const terser = require("gulp-terser");

src/js/package.yaml

@@ -3,11 +3,15 @@ scripts:
   minify: gulp
   fmt: cd ../.. && dprint fmt
   checkfmt: cd ../.. && dprint check
+  lint:
+    "cd ../.. && \
+    eslint --config ./.eslintrc.yml --ignore-path ./.eslintignore src/js src/web/skeleton"
 author: Emweb bv
 license : GPL-2.0-only
 private: true
 devDependencies:
   dprint: ^0.32.1
+  eslint: ^8.25.0
   gulp: ^4.0.2
   gulp-rename: ^2.0.0
   gulp-terser: ^2.1.0