add colored prompt

- user@host in bold green (bold red for root), directory in bold blue
- colors are disabled with NO_COLOR, TERM=dumb or when stdout is not a terminal
- parts are sanitized before adding the color codes
- only replace the home directory with ~ on a path boundary

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -22,6 +22,8 @@ Features:
     override with $GOSH_HISTFILE); lines starting with a space are not saved
   - Tab completion for commands and file paths
   - Tilde expansion (~, ~/path) at the start of unquoted words
+  - Colored prompt (user@host green, red for root; directory blue),
+    disabled with $NO_COLOR, TERM=dumb or when output is not a terminal
   - Environment variable expansion ($VAR, ${VAR}, ${VAR:-default});
     using an unset variable is an error, use ${VAR:-} for an empty default