Add barcode bar spacing / gap control

[productdevbook]

Context

• JsBarcode#412 — Increase spacing between bars? (4 comments)
• JsBarcode#431 — Allowing float values for bar width (1 comment)
• JsBarcode#402 — Width decimal is not supported? (2 comments)

Description

Users need finer control over barcode rendering:

1. Gap/spacing between bars for visual or scanning reasons
2. Fractional bar widths for precise physical sizing

Current Status in etiket

The barWidth option accepts a number and is used as a multiplier (barPixelWidth = w * barWidth), but there is no option to add spacing between bars and it's unclear if fractional values work correctly with the SVG coordinate system.

Proposed Solution

1. Add a barGap option (default: 0) that inserts space between bars
2. Ensure barWidth supports fractional values (e.g., 1.5, 0.75)
3. Update the SVG renderer to handle fractional coordinates cleanly
4. Document the relationship between barWidth and scanning reliability (too thin = scan failures)

[productdevbook]

Resolved in commit 85a3461 — added barGap option for bar spacing control. Tests added.