Add general-purpose functions for sizing in viewport units (#3833)

This PR adds general-purpose functions for obtaining a `Length` in
viewport units.

Previously in #3600 we had added specific variants of `w` and `h` that
took viewport units, but I think it makes more sense to just have a
primitive that can compose with any styling method that accepts a
`Length` or `Into<Length>`.

Release Notes:

- N/A

crates/storybook2/src/stories/viewport_units.rs

@@ -13,16 +13,16 @@ impl Render for ViewportUnitsStory {
                 .flex_row()
                 .child(
                     div()
-                        .w_vw(0.5, cx)
-                        .h_vh(0.8, cx)
+                        .w(vw(0.5, cx))
+                        .h(vh(0.8, cx))
                         .bg(gpui::red())
                         .text_color(gpui::white())
                         .child("50vw, 80vh"),
                 )
                 .child(
                     div()
-                        .w_vw(0.25, cx)
-                        .h_vh(0.33, cx)
+                        .w(vw(0.25, cx))
+                        .h(vh(0.33, cx))
                         .bg(gpui::green())
                         .text_color(gpui::white())
                         .child("25vw, 33vh"),