I'd like to add another custom eslint rule, but there isn't a very good place to do that right now. We have the `eslint-plugin-kibana-custom` package, which is super simple but isn't in the `@kbn` namespace and isn't included in the root eslint config, and `@kbn/eslint-plugin-license-header` is too specific, so I've merged those two packages into `@kbn/eslint-plugin-eslint`, which is a little redundant but allows is to refer to the rules within it as `@kbn/eslint/{rule}`, which feels nice.
Thoughts?
_**NOTE:**_ merging the eslint rules from the two packages means enabling prettier for the code from `@kbn/eslint-plugin-license-header`, all those changes are made in 42c7da6fe2. [View the changes without the prettier updates](b647f2b...74e07a0)
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