Merge ElementContext into WindowContext (#10979)

The new `ElementContext` was originally introduced to ensure the element
APIs could only be used inside of elements. Unfortunately, there were
many places where some of those APIs needed to be used, so
`WindowContext::with_element_context` was introduced, which defeated the
original safety purposes of having a specific context for elements.

This pull request merges `ElementContext` into `WindowContext` and adds
(debug) runtime checks to APIs that can only be used during certain
phases of element drawing.

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>

crates/ui/src/prelude.rs

@@ -2,9 +2,9 @@
 
 pub use gpui::prelude::*;
 pub use gpui::{
-    div, px, relative, rems, AbsoluteLength, DefiniteLength, Div, Element, ElementContext,
-    ElementId, InteractiveElement, ParentElement, Pixels, Rems, RenderOnce, SharedString, Styled,
-    ViewContext, WindowContext,
+    div, px, relative, rems, AbsoluteLength, DefiniteLength, Div, Element, ElementId,
+    InteractiveElement, ParentElement, Pixels, Rems, RenderOnce, SharedString, Styled, ViewContext,
+    WindowContext,
 };
 
 pub use crate::clickable::*;