tools: use 'readonly' for EventSource global

The EventSource entry was the only global in eslint.config.mjs still
using the deprecated 'readable' value. ESLint keeps 'readable' working
as an alias of 'readonly' for historical reasons, so this is a
consistency fix rather than a functional one.

The flat config migration converted the other globals in this block
but missed this single entry.

Fixes: #64757
Signed-off-by: Honey Tyagi <honeyt290@gmail.com>
PR-URL: #64787
Reviewed-By: Aviv Keller <me@aviv.sh>
Reviewed-By: Mike McCready <66998419+MikeMcC399@users.noreply.github.com>
Reviewed-By: René <contact.9a5d6388@renegade334.me.uk>

Author: HoneyTyagii

eslint.config.mjs

@@ -126,7 +126,7 @@ export default [
         CryptoKey: 'readonly',
         DecompressionStream: 'readonly',
         DisposableStack: 'readonly',
-        EventSource: 'readable',
+        EventSource: 'readonly',
         fetch: 'readonly',
         Float16Array: 'readonly',
         FormData: 'readonly',