✨ Allow configuring a vertical gutter separator #378

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.