Add editor::OpenUrl action and bind to gx in Vim mode (#7972)

This adds one feature I've been missing a lot in Vim mode: `gx` to open
the URL under the cursor.

Technically, in Vim, `gx` opens more "paths", not just URLs, but I think
this is a good start.

Release Notes:

- Added `gx` to Vim mode to open the URL under the cursor.

Demo:


https://github.com/zed-industries/zed/assets/1185253/6a19490d-b61d-40b7-93e8-4819599f6977

crates/editor/src/hover_links.rs

@@ -569,7 +569,7 @@ pub fn show_link_definition(
     editor.hovered_link_state = Some(hovered_link_state);
 }
 
-fn find_url(
+pub(crate) fn find_url(
     buffer: &Model<language::Buffer>,
     position: text::Anchor,
     mut cx: AsyncWindowContext,