Use twig template instead of render array

Problem/Motivation

Use a twig template for the layout builder block and add the "Browse Link" link to it as a variable instead of adding another render array to the $links array. This will make it easier to override when using hook_layout_builder_browser_alter().

Steps to reproduce

Proposed resolution

Use twig template instead of $link render array

Remaining tasks

Refactor in src/Controller/OverriddenBrowserController.php

User interface changes

none

API changes

none

Data model changes

none