Add grammar for `readonly` property modifier

[ghostwriter]

This patch adds the grammar for readonly property 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

property-modifier:
  'var'
-  visibility-modifier static-modifier?
-  static-modifier visibility-modifier?
+  visibility-modifier property-modifiers?
+  property-modifiers visibility-modifier?
+
+property-modifiers:
+  readonly-modifier
+  static-modifier
+
+readonly-modifier:
+  'readonly'

visibility-modifier:
  'public'

[ghostwriter]

or maybe, something like?

property-declaration:
 attributes? modifiers? types? property-elements ';'