Allow configuring the content width of the experimental Markdown editor

## Feature request

Please make the content width of the experimental hybrid Markdown editor configurable.

VS Code 1.131 introduced the new `Markdown Editor (Experimental)` (view type `vscode.markdown.editor`). When this editor is used in a wide editor group, the rendered document remains centered in a column with a maximum width of approximately 900 px. The rest of the editor width is left unused.

This differs from the existing Markdown Preview, which can use the available editor width. The fixed narrow column is especially limiting for wide tables, code blocks, diagrams, and documents designed to make use of a large display.

## Steps to reproduce

1. Use VS Code 1.131.0.
2. Open a `.md` file in a normal editor window.
3. Run **View: Reopen Editor With...** and select **Markdown Editor (Experimental)**.
4. Maximize the VS Code window or place the editor in a wide editor group.
5. Observe that the document content remains limited to roughly 900 px and is centered, leaving substantial unused space on both sides.

The same width limitation is present when the hybrid editor is opened in the Agents window, and in both Locked and Editing modes.

## Current implementation

In the bundled 1.131.0 Markdown editor code, the default `limitedWidth` value is `900`, and that value is applied to the `.md-editor-content` element as an inline `max-width`. There is currently no public setting that overrides it.

The existing `markdown.styles` setting only applies to the traditional Markdown Preview and does not affect `vscode.markdown.editor`.

## Proposed behavior

Add a user/workspace setting for the hybrid editor's maximum content width. For example, it could support:

- The current readable-width behavior as the default.
- A numeric pixel value for users who want a wider but still bounded document.
- A `full` or unlimited option that uses all available editor width, while retaining reasonable horizontal padding.

An example setting name could be `markdown.editor.maxContentWidth`, although the exact name and value format are not important to this request.

This should apply consistently in both normal editor windows and the Agents window.

## Why this is useful

A readable fixed width is a good default for prose, but Markdown is also commonly used for technical documents containing wide tables, long code blocks, Mermaid diagrams, and side-by-side comparisons. Allowing users to choose the width would preserve the current default while making the hybrid editor practical for these documents.