Ignore vscode-extension in root eslintignore (#4322)

Summary:
ernieturner originally had this line in #4275 but I thought it was safe to remove.

Because the `vscode-extension` is now not part of the Yarn workspace, its ESLint dependencies do not get installed by default. This means linting those files will fail (since the TypeScript-specific ESLint packages are not installed).

Here we add it back. The `vscode-extension` code gets linted with its own run. See https://github.com/facebook/relay/blob/7b5bee426ffc4fc5c9a3f9d06225020f7b7fe066/.github/workflows/ci.yml#L24

Pull Request resolved: #4322

Test Plan: See that this turns GitHub CI green again: https://github.com/facebook/relay/actions/runs/5050278394/jobs/9060793015?pr=4322

Reviewed By: josephsavona

Differential Revision: D46085331

Pulled By: captbaritone

fbshipit-source-id: 3c27c2a1db8ca94cf95ef70b5ba76b9ddb0721a0

.eslintignore

@@ -11,3 +11,5 @@ website-prototyping-tools/node_modules
 *.md
 src/flow/
 packages/relay-flight-experimental/
+# The VSCode Extension gets linted with a separate setup
+vscode-extension