Bump fork-ts-checker-webpack-plugin and notifier, update from babel-eslint to babel/eslint-parser

[daveajrussell]

Following #1983, a typeerror began emanating from the fork-ts-checker-webpack-plugin package during builds, due to the bump in the version of eslint. Whilst builds were still successfully producing artefacts, it's possible we'd no longer be notified of ts issues.

This updates fork-ts-checker-webpack-plugin to version 6.5.3.. version 7.0.0 and onward drop support for webpack 4 and eslint, which would require a much larger change. Also updated is the fork-ts-checker-notifier-webpack-plugin package to version 4.0.0.

The update to fork-ts-checker-webpack-plugin necessitated an upgrade from babel-eslint to @babel/eslint-parser due to an error emanating from eslint-scope during builds. The only necessary change from this was to update our eslint config to use @babel/eslint-parser instead.