Show color swatches for LSP completions (#18665)

Closes #11991

Release Notes:

- Added support for color swatches for language server completions.

<img width="502" alt="Screenshot 2024-10-02 at 19 02 22"
src="https://github.com/user-attachments/assets/57e85492-3760-461a-9b17-a846dc40576b">

<img width="534" alt="Screenshot 2024-10-02 at 19 02 48"
src="https://github.com/user-attachments/assets/713ac41c-16f0-4ad3-9103-d2c9b3fa8b2e">

This implementation is mostly a port of the VSCode version of the
ColorExtractor. It seems reasonable the we should support _at least_
what VSCode does for detecting color swatches from LSP completions.

This implementation could definitely be better perf-wise by writing a
dedicated color parser. I also think it would be neat if, in the future,
Zed handled _more_ color formats — especially wide-gamut colors.

There are a few differences to the regexes in the VSCode implementation
but mainly so simplify the implementation :
- The hex vs rgb/hsl regexes were split into two parts
- The rgb/hsl regexes allow 3 or 4 color components whether hsla/rgba or
not and the parsing implementation accepts/rejects colors as needed

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/editor/src/editor.rs

@@ -1228,6 +1228,10 @@ impl CompletionsMenu {
                                 None
                             };
 
+                        let color_swatch = completion
+                            .color()
+                            .map(|color| div().size_4().bg(color).rounded(px(2.)));
+
                         div().min_w(px(220.)).max_w(px(540.)).child(
                             ListItem::new(mat.candidate_id)
                                 .inset(true)
@@ -1243,6 +1247,7 @@ impl CompletionsMenu {
                                         task.detach_and_log_err(cx)
                                     }
                                 }))
+                                .start_slot::<Div>(color_swatch)
                                 .child(h_flex().overflow_hidden().child(completion_label))
                                 .end_slot::<Label>(documentation_label),
                         )