ESLint: enforce prefer-const where possible

https://eslint.org/docs/latest/rules/prefer-const

In our tests we use this pattern a lot:
```
let something;

beforeEach(() => {
  something = ...;
});
```

This is not valid ~ prefer-const, but can be "let through" with some configuration.
However, this would make the rule too loose for the non-test codebase.

Therefore, we enable it overall but turn it off for tests only -- the `*.spec.ts` overrides apply to test files only and take precedence over `*.ts`

.eslintrc.json

@@ -104,7 +104,7 @@
             "allowTernary": true
           }
         ],
-        "prefer-const": "off",            // todo: re-enable & fix errors (more strict than it used to be in TSLint)
+        "prefer-const": "error",
         "prefer-spread": "off",
         "no-underscore-dangle": "off",
 
@@ -213,6 +213,21 @@
         ]
       }
     },
+    {
+      "files": [
+        "*.spec.ts"
+      ],
+      "parserOptions": {
+        "project": [
+          "./tsconfig.json",
+          "./cypress/tsconfig.json"
+        ],
+        "createDefaultProgram": true
+      },
+      "rules": {
+        "prefer-const": "off"
+      }
+    },
     {
       "files": [
         "*.html"