fix chezmoi segment and rename it to chezmoi_shell (#2311)

- Link to the project's homepage rather than its source code.
- Move `chezmoi_shell` next to all the other *shell indicator* segments.
- Use a shade of blue that resembes the color on chezmoi.io.
- Don't go beyond 8 colors in 8-color configs.
- Remove the segment from the *pure* config.
- Fix whitespace padding on `CHEZMOI_ICON`.
- Use the appropriate icon with all fonts (the same as `HOME`).
- Add missing `CHEZMOI_ICON` definitions for "powerline" and "ascii" font modes.
- Remove the redundant literal "chezmoi" content from the segment.
- Fix instant prompt so that the segment is shown only when `$CHEZMOI` is non-empty.

config/p10k-pure.zsh

@@ -51,7 +51,6 @@
     newline                   # \n
     virtualenv                # python virtual environment
     prompt_char               # prompt symbol
-    chezmoi                   # chezmoi prompt (https://github.com/twpayne/chezmoi)
   )
 
   # Right prompt segments.