fix(line-numbers): fixes to line numbers column

[metonym]

A couple of fixes:

• Line numbers should not be covered by a solid fill, custom line background color
• Line numbers column should not be covered by code if scrollable

[render[bot]]

Your Render PR Server URL is https://svelte-highlight-pr-271.onrender.com.

Follow its progress at https://dashboard.render.com/static/srv-cgk6mf5269v743c71fjg.

[render[bot]]

Your Render PR Server URL is https://svelte-highlight-pr-271-mmex.onrender.com.

Follow its progress at https://dashboard.render.com/static/srv-cgk6mfd269v743c71fng.