Update panel.focused_border token across themes (#21612)

Follow up to https://github.com/zed-industries/zed/pull/21593

This PR updates all built-in themes `panel.focused_border` tokens using
the same HEX code used for `text_accent`.

There shouldn't be any visual change here given the project panel item,
when focused, was using `Color::Selected`, which maps to `text_accent`,
to color its border. In the linked PR above, the project panel item was
updated to use the dedicated token for that. This is good because now
theme markers will be able to customize them separately (e.g., having a
different `text_accent` color than `panel.focused_border`).

Release Notes:

- N/A

assets/themes/ayu/ayu.json

@@ -46,7 +46,7 @@
         "tab.active_background": "#0d1016ff",
         "search.match_background": "#5ac2fe66",
         "panel.background": "#1f2127ff",
-        "panel.focused_border": null,
+        "panel.focused_border": "#5ac1feff",
         "pane.focused_border": null,
         "scrollbar.thumb.background": "#bfbdb64c",
         "scrollbar.thumb.hover_background": "#2d2f34ff",
@@ -416,7 +416,7 @@
         "tab.active_background": "#fcfcfcff",
         "search.match_background": "#3b9ee566",
         "panel.background": "#ececedff",
-        "panel.focused_border": null,
+        "panel.focused_border": "#3b9ee5ff",
         "pane.focused_border": null,
         "scrollbar.thumb.background": "#5c61664c",
         "scrollbar.thumb.hover_background": "#dfe0e1ff",