Unify definition of aliases in single place.
Make TypeScript configuration file (`tsconfig.json`) source of truth
regarding aliases.
Both webpack (through `vue.config.js`) and ESLint (through
`.eslintrc.js`) now reads the alias configuration from `tsconfig.json`.
- Use function abstractions (such as map, reduce, filter etc.) over
for-of loops to gain benefits of having less side effects and easier
readability.
- Enable `downLevelIterations` for writing modern code with lazy evaluation.
- Refactor for of loops to named abstractions to clearly express their
intentions without needing to analyse the loop itself.
- Add missing cases for changes that had no tests.
Alias would remove unnecessary repetitions and less relative paths make changes easier when moving around files. This commit cleans also up some relative paths ('../../../') by using the alias and orders imports. It updates both path alias in tsconfig and module alias in Vue CLI's bundler (vuejs/vue-cli#2398).
