add xterm color names to ansi --list (#7513)

# Description

Follow up to #7141 to map @webbedspace's rgb colors to xterm 256 color
indexes. Also added the xterm 256 named colors to `ansi --list` in the
process.

The few dozen or so names that were duplicate in the xterm 256 names
from [here](https://www.ditig.com/256-colors-cheat-sheet) were renamed
by appending a,b,c,d. So, instead of two blue3's there will be blue3a
and blue3b.

# User-Facing Changes

More colors.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-utils/src/sample_config/default_config.nu

@@ -228,23 +228,23 @@ let light_theme = {
       } else { 'blue_bold' }
     }
     duration: dark_gray
-    date: { (date now) - $in |
-      if $in < 1hr {
-        '#e61919'
-      } else if $in < 6hr {
-        '#e68019'
-      } else if $in < 1day {
-        '#e5e619'
-      } else if $in < 3day {
-        '#80e619'
-      } else if $in < 1wk {
-        '#19e619'
-      } else if $in < 6wk {
-        '#19e5e6'
-      } else if $in < 52wk {
-        '#197fe6'
-      } else { 'dark_gray' }
-    }
+  date: { (date now) - $in |
+    if $in < 1hr {
+      'red3b'
+    } else if $in < 6hr {
+      'orange3'
+    } else if $in < 1day {
+      'yellow3b'
+    } else if $in < 3day {
+      'chartreuse2b'
+    } else if $in < 1wk {
+      'green3b'
+    } else if $in < 6wk {
+      'darkturquoise'
+    } else if $in < 52wk {
+      'deepskyblue3b'
+    } else { 'dark_gray' }
+  }
     range: dark_gray
     float: dark_gray
     string: dark_gray