Introduce the Source Editor Toolbar

What does this MR do and why?

This is the first out of two MRs introducing the toolbar into Source Editor:

• Introduce the Toolbar and its integration into the raw Source Editor (this MR)
• Introduce the UI components for the toolbar and connect those to the editor

This MR does the following:

• Introduces the Toolbar class with only add and remove methods for now. The instance of the Toolbar class will be created with the items property containing two groups of the toolbar items: left and right (the strings are defined as constants). The toolbar instance also creates an event emitter (in Monaco way) that gets fired every time we update the toolbar items.
• Introduces the very basic Button class to be shared among the toolbar items. Later we might need to add other types like toggle to be addable to the toolbar
• We connect the toolbar and Source Editor via the .toolbar property on the instance for it to be available anywhere the instance is available
• We make use of the toolbar in the markdown live preview extension by adding the live preview button

How to set up and validate locally

Unfortunately, without the second part (UI integration) there is no easy way to check that a button is added to the toolbar, but the tests in this MR provide extensive coverage for different modules and scenarios.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #280800 (closed)