Add grammar for `readonly` class modifier

[ghostwriter]

This patch adds the grammar for readonly class modifier, via the following changes:

• Update spec/14-classes.md
• Execute tools/pre-commit

diff --git a/spec/14-classes.md b/spec/14-classes.md

--- a/spec/14-classes.md
+++ b/spec/14-classes.md

 class-declaration:
-  class-modifier? 'class' name class-base-clause? class-interface-clause? '{' class-member-declarations? '}'
+  class-modifiers? 'class' name class-base-clause? class-interface-clause? '{' class-member-declarations? '}'
+
+class-modifiers:
+  class-modifier
+  class-modifiers class-modifier

 class-modifier:
   'abstract'
   'final'
+  'readonly'