Fix highlighting in picker with multiple columns

[exp80]

Fixes #6254

This could be probably done in a less hacky way, but I didn't want to overcomplicate things.

[the-mikedavis]

Adding to cell_start_byte_offset in the None branch is a nice catch and that should stay 👍. The other change is not the right fix though. The offset is caused by Row::cell_text appending newlines to each cell's text and that should be fixed directly instead. It's caused by a bug introduced in b2837ff in impl<'a> From<&Text<'a>> for String where each line gets a newline appended. That newline should only be added between lines though. There's a small refactor of that for that fixes the issue:

for spans in &text.lines {
    if !output.is_empty() {
        output.push('\n');
    }
    for span in &spans.0 {
        output.push_str(&span.content);
    }
}

[exp80]

Thanks, I didn't know if that was intentional or not. Fixed.