Take `babel-eslint`'s move to `@babel/eslint-parser`.

[chrisbobbe]

In July, Babel made an announcement:

With the next iteration of babel-eslint, we have decided to publish the package under a new name: @babel/eslint-parser.

We may also like to consider @babel/eslint-plugin, the new home of eslint-plugin-babel. For one data point, reportedly, this plugin would help us suppress false positives from ESLint's no-unused-expressions rule centrally and precisely, where such false positives occur when using method calls with optional chaining.

Blocked by the ESLint 7 upgrade (#4253) because @babel/eslint-parser does not have a version on NPM that supports ESLint 6. The same is true of @babel/eslint-plugin.