themes: Swap ANSI white with ANSI black (#32175)

Here’s how it looks after the fix:
![colors](https://github.com/user-attachments/assets/11c78ad6-da50-4aad-b133-9be5e3844878).
White is white and black is black, as intended.

In some cases, dimmed colors were poorly defined, so I took
`text.dimmed` values.

Note that white is defined exactly as the background color for light
themes. Similarly, black is the exact background color for dark themes.
I didn’t change this, but many themes intentionally make white and black
slightly different from the background color. This prevents issues where
programs assume, say, a dark background and set the foreground to white,
making text invisible. I'm not sure if we want to adjust these themes to
address this; just noting it here.


Closes #29379 

Release Notes:

- Fixed ANSI black and ANSI white colors in built-in themes

assets/themes/ayu/ayu.json

@@ -442,9 +442,9 @@
         "terminal.foreground": "#5c6166ff",
         "terminal.bright_foreground": "#5c6166ff",
         "terminal.dim_foreground": "#fcfcfcff",
-        "terminal.ansi.black": "#fcfcfcff",
-        "terminal.ansi.bright_black": "#bcbec0ff",
-        "terminal.ansi.dim_black": "#5c6166ff",
+        "terminal.ansi.black": "#5c6166ff",
+        "terminal.ansi.bright_black": "#3b9ee5ff",
+        "terminal.ansi.dim_black": "#9c9fa2ff",
         "terminal.ansi.red": "#ef7271ff",
         "terminal.ansi.bright_red": "#febab6ff",
         "terminal.ansi.dim_red": "#833538ff",
@@ -463,9 +463,9 @@
         "terminal.ansi.cyan": "#4dbf99ff",
         "terminal.ansi.bright_cyan": "#ace0cbff",
         "terminal.ansi.dim_cyan": "#2a5f4aff",
-        "terminal.ansi.white": "#5c6166ff",
-        "terminal.ansi.bright_white": "#5c6166ff",
-        "terminal.ansi.dim_white": "#9c9fa2ff",
+        "terminal.ansi.white": "#fcfcfcff",
+        "terminal.ansi.bright_white": "#fcfcfcff",
+        "terminal.ansi.dim_white": "#bcbec0ff",
         "link_text.hover": "#3b9ee5ff",
         "conflict": "#f1ad49ff",
         "conflict.background": "#ffeedaff",