Feature: Custom gutter columns for per-line metadata (blame, annotations)

[gimenete]

Prerequisites

• I have searched for duplicate or closed feature requests
• I have read the contributing guidelines

Proposal

Add support for custom gutter columns that display per-line metadata alongside code content.

This could be exposed as a renderGutter callback or a gutters prop:

// Option A: Simple callback
<File
  file={file}
  renderGutter={(lineNumber: number) => (
    <span className="blame">{blameData[lineNumber].author}</span>
  )}
/>

// Option B: Multiple gutters
<File
  file={file}
  gutters={[
    { id: 'blame', width: 180, render: (line) => <BlameInfo line={line} /> }
  ]}
/>

This would enable displaying git blame information (author, commit hash, date), inline linting/CI
annotations, and custom line markers.

Motivation and context

When building code review or repository browsing tools, users often need to see git blame data alongside each line. Currently there's no way to render persistent per-line content in a gutter column:

• Annotations render as blocks below lines, not inline
• renderHoverUtility only shows on hover, not persistently
• unsafeCSS can style lines but cannot inject text content

The workaround is building a parallel column outside the component with synchronized scrolling, which is complex due to Shadow DOM encapsulation.

This is a common feature in code editors (VS Code, GitHub, GitLab) and would expand the library's use cases for code analysis and review tools.

[amadeus]

This is definitely something we need to add, it probably won't come until after virtualization and the new decoration API however