Migrate Positron ESLint configuration from .eslintrc.json to eslint.c…

…onfig.js (#6172)

As part of merging 1.96 from upstream, ESLint was switched to use flat
config (see: `"eslint.useFlatConfig": true` in `.vscode/settings.json`).
As a result, React linting from `eslint-plugin-react` and
`eslint-plugin-react-hooks` were no longer configured for Positron.

This PR migrates Positron-specific ESLint configuration settings from
`.eslintrc.json` to `eslint.config.js`, thus reestablishing React
linting (most importantly, rules for `react-hooks/rules-of-hooks` and
`react-hooks/exhaustive-deps`).

So, you will now be informed when your React hooks are missing
dependencies again:

<img width="1009" alt="Pasted Graphic"
src="https://github.com/user-attachments/assets/b0ffc323-9725-4ee3-a890-05e6710b353a"
/>