tools: use 'readonly' for EventSource global

[HoneyTyagii]

The EventSource entry in eslint.config.mjs was the only global still using the deprecated readable value, while the other 40+ entries in the same block use readonly.

ESLint keeps readable and writeable working only for historical reasons and documents them as deprecated in favour of readonly and writable, so behavior is unchanged. This is a consistency and future-proofing fix.

The inconsistency dates back to the flat config migration in 7e6d92c ("tools: update ESLint to v9 and use flat config"), which converted the other globals from the old .eslintrc.js spelling but missed this single entry.

Verified locally: eslint.config.mjs, lib, benchmark, tools, and the two tests that reference the global (test/parallel/test-eventsource.js and test-eventsource-disabled.js) all lint clean, so no-undef still resolves EventSource as expected.

Fixes: #64757

[MikeMcC399]

Deprecation reference in https://eslint.org/docs/latest/use/configure/language-options#use-configuration-files

[HoneyTyagii]

Thanks for the review @MikeMcC399, and for the deprecation link, that's a better reference than the ESLint PR I cited in the issue.
Good catch on the DCO, I missed it. Amended with git commit -s --amend and force pushed, so the Signed-off-by line is now present, and lint-commit-message should pass on the re-run. The diff itself is unchanged, still the single line in eslint.config.mjs.

[HoneyTyagii]

You're right, @MikeMcC399, sorry for the noise. The amend was local, the push didn't go through on my end, so the branch is still at 7e505b4. Redoing it now, and I'll confirm here once the signed-off commit actually shows up on the PR.

[MikeMcC399]

Please change

Refs: #64757

to

Fixes: #64757

because this PR fixes the issue #64757 that you originally opened. When this PR lands, then the issue will be closed automatically. If you only write Refs: the issue will need to be closed manually afterwards.

See also https://github.com/nodejs/node/blob/main/doc/contributing/pull-requests.md#commit-message-guidelines I linked to before.

[HoneyTyagii]

Pushed as e3b900d. Switched Refs: to Fixes: so the issue closes on landing, and the commit now carries the Signed-off-by line. Thanks for walking me through it. The diff is unchanged, still the single line in eslint.config.mjs.

[MikeMcC399]

I'm sorry I wasn't very clear. The Fixes: statement usually goes into the PR description and is added automatically into the commit message.

I've edited it manually to speed things up.

[MikeMcC399]

GitHub links the PR and the issue automatically now