Code blocks: UI to remove line selection

[feasgal]

Context

No response

Description

I'd really like to be able to change my mind about highlighting any lines at all, without reloading the page. Currently, once I toggle the line selection on and select a line, I can only move the selection, not remove it.

Related links

I mentioned this in comments to discussion#5195.

Use Cases

This seems like something anyone who uses the line selection would want.

Visuals

I'm not sure what it would look like for UI, but my instinct was to click an already-selected line.

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