✨ Allow configuring a vertical gutter separator

### Description

Please allow API consumers to configure an optional vertical separator between the line-number gutter and the editor content.

Some editor designs use a thin solid or dashed line beside the line numbers. The separator needs to be drawn by the gutter itself because the gutter width changes as the document's line-number digit count grows. There is currently no public configuration for this, so matching such a design requires maintaining a fork of `GutterView`.

A backward-compatible configuration could keep the separator disabled by default and allow consumers to specify its color and drawing style. The exact API is open to the maintainers' preference.

### Alternatives Considered

- Overlaying a line outside `SourceEditor`. This cannot reliably follow the gutter because its width is managed internally and changes with the number of line-number digits.
- Making internal gutter layout properties public. A configuration option would expose less implementation detail.

### Additional Context

Our current use case is a 1 pt separator using `NSColor.separatorColor` with a `[1, 1]` dash pattern.

No matching open or closed issue was found before filing this request. Observed on current `main` at revision `1fa4d3c`.

### Screenshots

Not included.

Start by reading GutterView and the surrounding SourceEditor gutter layout and drawing code. Trace how gutter width changes with line-number digits, then define a backward-compatible optional separator with configurable color and drawing style; done means it remains disabled by default and follows the gutter boundary as its width changes.

索引モデルが issue の本文から書いたものです。