feat: markdown-basics snippets: quote all lines

Previously, this snippet just inserted "> " before the selected text, which is
not very useful; particularly, when multiple paragraphs are selected, this will
only quote the first one.

Instead, we now prefix every line in the selection with "> ", so that it will
all be quoted regardless of how many paragraphs are taken.

This does also change the behavior in the case of no selected text slightly;
previously it would insert "> []" (where "[]" indicates the selection; that is,
it would put the insertion point after the quote marker). It now inserts
"[> ]" instead (that is, the same text but it's all selected) so if you start
typing you'll wind up removing the quote marker. Since using this snippet *not*
on a selection seems like a roundabout way of writing "> " this seems like an
acceptable tradeoff.