Add a right-side fixed per-line annotation gutter API for extensions

## Problem

VS Code extensions can display per-line annotations using text editor decorations.
However, `after` decorations are rendered immediately after each line's content,
so their horizontal position varies with line length.

This prevents extensions from presenting a stable right-side column for
per-line metadata, comparable to the existing left editor gutter.

## Use case

IBM i source members traditionally show their six-digit source date in a fixed
column at the far right of every line. The Code for IBM i extension can show
these dates in the left gutter today, but cannot reproduce the familiar
right-side layout without changing document text.

A stable right-side annotation gutter would also be useful for other
fixed-width, mainframe, legacy, and metadata-oriented source formats.

## Proposal

Expose a public extension API for a fixed right-side per-line annotation gutter,
parallel to the existing left-side editor gutter.

The API should allow extensions to:
- Render text or icons per line in a fixed right-side column.
- Define width, alignment, theme-aware colors, hover messages, and commands.
- Respect editor layout features such as word wrap, minimap, diff editors,
sticky scroll, and accessibility settings.
- Avoid altering document content or affecting cursor positions and edits.

## Current workaround

Using `TextEditorDecorationType.after` does not solve this because annotations
follow variable line length rather than occupying a fixed column.