[SPIKE] switch to typescript-tslint-plugin

[bajtos]

Replace a standalone tslint build step with configuring tslint as a TypeScript language service plugin. See https://github.com/Microsoft/typescript-tslint-plugin

Acceptance criteria

• Find if there are any features that we rely on and that are not supported by typescript-tslint-plugin
• Compare the performance. E.g. run time npm test before and after the switch and check the time difference.
• Find out how to upgrade @loopback/build and asses impact of such changes on existing projects. E.g. if we merge tslint.json with tslint.build.json, then I suspect we will break all existing projects.
• Review the todo list below and add any missing steps.
• Review DEVELOPING.md and indentify places that will need an update.

A preliminary task list for the real work:

• npm run build runs tslint as part of the compilation step
• all tslint-related tasks (tslint, tslint:fix) are removed
• tslint.config and tslint.build.config are merged into a single configuration. This applies to all packages in our monorepo.
• Instructions for VSCode users are telling developers to disable vscode-tslint extension.
• New projects scaffolded via lb4 will use typescript-tslint-plugin too.
• A blog post to publicize this change. Explain what is changing for LB contributors, what is changing for LB app developers, what (if any) migration is required for existing projects.

[bajtos]

This spike is no longer relevant, we have replaced tslint with typescript-eslint, see #2492