Add linting using eslint with standard as the base

Where possible we want to enforce the coding standards - https://github.com/DEFRA/water-abstraction-team/blob/main/coding_conventions.md

This change introduces eslint using standard as a base (enforcing all the standard js rule). Switching to eslint allows us to extend standard js.

A new command is added to only lint changed files. This will allow us to gradually change and enforce the new linting rules.

.eslintrc.js

@@ -0,0 +1,34 @@
+module.exports = {
+  extends: 'standard', // Maintain Standard.js rules
+  rules: {
+    'max-len': ['error', {
+      code: 120
+    }]
+  }
+}
+
+/*
+
+This is the code to enforce agreed conventions
+
+npm packages needed -
+    "eslint-plugin-require-sort": "^1.3.0",
+    "eslint-plugin-sort-keys": "^2.3.5",
+
+module.exports = {
+  extends: 'standard', // Maintain Standard.js rules
+  // ignorePatterns: ['test/*'], // we can ignore patterns to allow to gradually enforce lints
+  plugins: ['sort-keys', 'require-sort'],
+  rules: {
+    'func-style': ['error', 'declaration'],
+    'max-len': ['error', {
+      code: 120
+    }],
+    'require-sort/require-sort': 'error',
+    'sort-keys': [
+      'error', 'asc', { caseSensitive: false }
+    ]
+  }
+}
+
+*/