Markdown editor: Add discoverable block authoring affordances

Testing #326540

### Description

The experimental Markdown editor renders structured blocks such as headings, lists, task lists, blockquotes, and fenced code blocks, but does not provide a discoverable way to insert or convert these block types. A user who is unfamiliar with Markdown syntax has no visible pointer affordance, contextual action, or command-discovery path for creating them.

This makes the visual editor approachable for reading existing Markdown while leaving basic authoring dependent on prior syntax knowledge. The request is for accessible keyboard and pointer-based discovery of the supported block operations, without prescribing a specific UI.

### Reproduction

1. Open a Markdown document in the experimental Markdown editor and switch to **Editing** mode.
2. As a user unfamiliar with Markdown syntax, place the caret on an empty line or in a paragraph.
3. Attempt to create or convert the block to a heading, task list, blockquote, or fenced code block without typing Markdown marker syntax.
4. Check the visible editor UI and available contextual actions for an insertion or conversion path.

### Expected

Supported block types can be discovered and created through accessible keyboard and pointer affordances or command discovery. The exact interaction could follow existing VS Code patterns and does not need to be a particular menu or toolbar.

### Actual

There is no discoverable editor UI path for inserting or converting these block types. Users must know and type the corresponding Markdown syntax.