feat(eslint-plugin): [prefer-readonly] private fields support (#7686)

* Add private field support to prefer-readonly

* Fix change in prefer-readonly docs

* Restore missing test

* Update docs to mention private fields

* Update packages/eslint-plugin/docs/rules/prefer-readonly.md

---------

Co-authored-by: Josh Goldberg ✨ <git@joshuakgoldberg.com>

packages/eslint-plugin/docs/rules/prefer-readonly.md

@@ -6,7 +6,7 @@ description: "Require private members to be marked as `readonly` if they're neve
 >
 > See **https://typescript-eslint.io/rules/prefer-readonly** for documentation.
 
-Member variables with the privacy `private` are never permitted to be modified outside of their declaring class.
+Private member variables (whether using the `private` modifier or private `#` fields) are never permitted to be modified outside of their declaring class.
 If that class never modifies their value, they may safely be marked as `readonly`.
 
 This rule reports on private members are marked as `readonly` if they're never modified outside of the constructor.
@@ -22,6 +22,7 @@ class Container {
   // These member variables could be marked as readonly
   private neverModifiedMember = true;
   private onlyModifiedInConstructor: number;
+  #neverModifiedPrivateField = 3;
 
   public constructor(
     onlyModifiedInConstructor: number,
@@ -49,6 +50,13 @@ class Container {
   public mutate() {
     this.modifiedLater = 'mutated';
   }
+
+  // This is modified later on by the class
+  #modifiedLaterPrivateField = 'unchanged';
+
+  public mutatePrivateField() {
+    this.#modifiedLaterPrivateField = 'mutated';
+  }
 }
 ```
 

packages/eslint-plugin/src/rules/prefer-readonly.ts

@@ -287,7 +287,10 @@ class ClassScope {
 
   public addDeclaredVariable(node: ParameterOrPropertyDeclaration): void {
     if (
-      !tsutils.isModifierFlagSet(node, ts.ModifierFlags.Private) ||
+      !(
+        tsutils.isModifierFlagSet(node, ts.ModifierFlags.Private) ||
+        node.name.kind === ts.SyntaxKind.PrivateIdentifier
+      ) ||
       tsutils.isModifierFlagSet(node, ts.ModifierFlags.Readonly) ||
       ts.isComputedPropertyName(node.name)
     ) {