Convert to eslint 9

[robertbrignull]

Upgrades eslint to version 9, as well as upgrading a few plugins.

I've tried to convert the old config as closely as I could, but I've removed a few plugins that weren't working well because they haven't received updates in a couple of years. It's also very possible I've misunderstood bits of the config!

I can now at least run npm run lint locally, though it generates a lot of errors. Currently 1346 errors! I haven't worked out yet exactly where they're coming from but I think the majority are updates to the recommended configs and are mostly in the test code. Possibly there are some extra rules we want disable on test code, particularly around use of any and other unsafe access. There are also some we probably want to fix.

I wanted to put up a PR with what I have, and either I or someone else can take a look at the set of errors and see what to do.

[Copilot]

Pull Request Overview

This PR upgrades ESLint to version 9, replaces the legacy configuration with a new flat config (eslint.config.mjs), and updates the lint/format scripts and related dependencies.

• Upgraded ESLint and related plugins to v9.
• Simplified format/lint/lint-ci scripts in package.json.
• Introduced eslint.config.mjs and removed .eslintrc.js and .eslintignore.

Reviewed Changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 2 comments.

File	Description
extensions/ql-vscode/package.json	Bumped ESLint/plugins versions; removed explicit --ext flags
extensions/ql-vscode/eslint.config.mjs	Added new flat ESLint config leveraging @eslint/js
extensions/ql-vscode/.eslintrc.js	Removed old ESLint config
extensions/ql-vscode/.eslintignore	Removed old ignore file

Comments suppressed due to low confidence (1)

extensions/ql-vscode/eslint.config.mjs:84

• The projectService parser option is not documented for @typescript-eslint/parser; it may not have any effect. Consider removing it or using only the supported project setting.

        projectService: true,

[Copilot]

The lint script no longer specifies file extensions, so TypeScript files (.ts, .tsx) may be skipped. Consider adding --ext .js,.ts,.tsx to ensure all relevant files are linted.

Suggested change

	"lint": "eslint . --max-warnings=0",
	"lint": "eslint . --max-warnings=0 --ext .js,.ts,.tsx",

[Copilot]

Importing from typescript-eslint seems incorrect; the official plugin package is @typescript-eslint/eslint-plugin. Update this import to ensure the TypeScript ESLint rules are applied correctly.

Suggested change

	import tseslint from "typescript-eslint";
	import tseslint from "@typescript-eslint/eslint-plugin";