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.

README.md

@@ -314,7 +314,7 @@ enable as many segments as you like. It won't slow down your prompt or Zsh start
 | `azure` | [azure](https://docs.microsoft.com/en-us/cli/azure) account name |
 | `background_jobs` | presence of background jobs |
 | `battery` | internal battery state and charge level (yep, batteries *literally* included) |
-| `chezmoi` | [chezmoi](https://github.com/twpayne/chezmoi) shell |
+| `chezmoi_shell` | [chezmoi](https://www.chezmoi.io/) shell |
 | `command_execution_time` | duration (wall time) of the last command |
 | `context` | user@hostname |
 | `cpu_arch` | CPU architecture |