Refactored grouping of SE buttons

What does this MR do and why?

1. The MR introduces the third group for the Source Editor toolbar buttons as per Support 3 areas for the buttons on the Source E... (#385895 - closed)
2. The MR renames the groups to represent their purpose/functionality instead of placement

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. In rails console enable the toolbar either globally, or for your user

   Feature.enable(:source_editor_toolbar)

2. In a repository, open a Markdown file. Alternatively, create a new file and give it a name of something like foo.md: the toolbar will be shown automatically once you focus out of the file name field

3. Observe the edit group buttons (buttons editing the actual content like bold, italic, etc.) are still correctly placed on the left, while the settings buttons (soft wrap, live preview) are still placed on the right as it is shown on the screenshots

⚠ we do not yet have buttons for the file group so that's why there're no visual changes. The group will be populated in the course of Source Editor: Multi file context (&7005 - closed)

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 #385895 (closed)