Add element_selection_background highlight to theme (#32388)

Closes #32354

The issue is that we render selections over the text in the agent panel,
but under the text in editor, so themes that have no alpha for the
selection background color (defaults to 0xff) will just occlude the
selected region. Making the selection render under the text in markdown
would be a significant (and complicated) refactor, as selections can
cross element boundaries (i.e. spanning code block and a header after
the code block).

The solution is to add a new highlight to themes
`element_selection_background` that defaults to the local players
selection background with an alpha of 0.25 (roughly equal to 0x3D which
is the alpha we use for selection backgrounds in default themes) if the
alpha of the local players selection is 1.0. The idea here is to give
theme authors more control over how the selections look outside of
editor, as in the agent panel specifically, the background color is
different, so while an alpha of 0.25 looks acceptable, a different color
would likely be better.

CC: @iamnbutler. Would appreciate your thoughts on this. 

> Note: Before and after using Everforest theme

| Before | After | 
|-------| -----|
| <img width="618" alt="Screenshot 2025-06-09 at 5 23 10 PM"
src="https://github.com/user-attachments/assets/41c7aa02-5b3f-45c6-981c-646ab9e2a1f3"
/> | <img width="618" alt="Screenshot 2025-06-09 at 5 25 03 PM"
src="https://github.com/user-attachments/assets/dfb13ffc-1559-4f01-98f1-a7aea68079b7"
/> |

Clearly, the selection in the after doesn't look _that_ great, but it is
better than the before, and this PR makes the color of the selection
configurable by the theme so that this theme author could make it a
lighter color for better contrast.




Release Notes:

- agent panel: Fixed an issue with some themes where selections inside
the agent panel would occlude the selected text completely

Co-authored-by: Antonio <me@as-cii.com>

crates/ui_prompt/src/ui_prompt.rs

@@ -153,7 +153,10 @@ impl Render for ZedPromptRenderer {
                         });
                         MarkdownStyle {
                             base_text_style,
-                            selection_background_color: { cx.theme().players().local().selection },
+                            selection_background_color: cx
+                                .theme()
+                                .colors()
+                                .element_selection_background,
                             ..Default::default()
                         }
                     }))