[eslint] require final newline (#32265)

We have had `insert_final_newline = true` in our `.editorconfig` file since April 2015, but never validated it with eslint. I'm a little tired of extra changes showing up in PRs because some people use editors which respect the `.editorconfig` file and some don't, so I figured we might want to enable the rule in eslint so that the can be autofixed.

Votes requested please :)

packages/eslint-config-kibana/.eslintrc.js

@@ -100,6 +100,7 @@ module.exports = {
     strict: [ 'error', 'never' ],
     'valid-typeof': 'error',
     'wrap-iife': [ 'error', 'outside' ],
+    'eol-last': ['error', 'always'],
     yoda: 'off',
 
     'object-curly-spacing': 'off', // overridden with babel/object-curly-spacing