In Source Editor, the markdown buttons moved to the left

What does this MR do and why?

The MR places the buttons introduced by EditorMarkdownExtension in Source Editor's toolbar to the left instead of right. This is to separate the editing actions (bold, italic, etc.) from the editor-wide actions (like Live Preview) as per #385895 (comment 1241725047)!

⚠ The whole toolbar effort is behind the feature flag, hence no changelog is provided.

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 content editing buttons correctly placed on the left, instead of on the right of the toolbar

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)