Switch to ESLint v9 and flat config (#3558)

Since PR #3551 was not yet complete, I made my own attempt.

1. Update to ESLint v9.
2. Replace deprecated `.eslintrc.json` and `.eslintignore` by flat
config `eslint.config.mjs`.
3. Adapt `check_config.js` to use flat config.
4. Since `eslint-plugin-import` still doesn't support ESLint v9 I
removed it. We can add it back when it does support v9.
5. Run tests `npm run check:js` and `npm run config:check`.
6. In order not to overload this PR, I have not yet activated more
additional rules - there are some useful ones in the new plugin
`@eslint/js`.

@bugsounet, please don't take it as an offence that I have created a
competing PR. The migration to ESLint v9 has been burning under my nails
for some time.

tests/unit/functions/server_functions_spec.js

@@ -19,7 +19,7 @@ describe("server_functions tests", () => {
 				},
 				text: fetchResponseHeadersText
 			};
-			// eslint-disable-next-line
+
 			fetch = jest.fn();
 			fetch.mockImplementation(() => fetchResponse);