fix: throw TypeError on fixtures mutation (#147)

Author: mondeja

README.md

@@ -318,8 +318,8 @@ Fixtures are used to inject data for each linted file. It should be a function
 that takes the same parameters as a custom rule, but it returns an object with
 the data to inject. The data can be accessed in the custom rules.
 
-Fixture values are mutable as the fixtures object is passed as reference to
-not create unnecessary copies of the data, so rules should not mutate them.
+Fixture values are inmutable, a `TypeError` will be thrown if you try to modify
+them in a custom rule.
 
 #### Example
 
@@ -343,15 +343,6 @@ const config = {
                 /** @type {Info} */ {fixtures: {iconPath, segments, bbox}},
             ) => {
                 reporter.name = 'my-custom-rule';
-
-                reporter.warn(`Segments: ${segments}`);
-
-                // WARNING: don't do this
-                // fixtures.myFixture = 'new value';
-                //
-                // Instead, create a deep clone
-                // const myFixture = globalThis.structuredClone(fixtures.myFixture);
-                // myFixture.newProperty = 'new value';
             }
         ]
     },