Transform landing into editor type plugin

[juancho0202]

Currently we add custom code to our Layout for the landing page of open-scd (Open Project, New Project buttons). We would like to make this an editor type plugin that is visible by default if no project is loaded.

Acceptance Criteria:

• A new editor type plugin is added to packages/plugins and in the plugins.js file it should have requiredDoc: false
• When OpenSCD loads and no doc is opened you should see the mwc-tab bar with a first option called "Start" which includes the same code we currently have for the landing page, then a list of editor type plugins that don't require a doc.