feat(hostname): add detect_env_vars as option (#5196)

* Added hostname.detect_env_vars

based on the newly added context::detect_env_vars

- extended context::detect_env_vars to check for negated environment
  variables as well, analogous to the other detect modules
- made hostname.detect_env_vars only active if ssh_only is set to false
  for backwards compatibility

Co-authored-by: Dominik Sander <dsander@users.noreply.github.com>

* added clippy recommendations, removed unneeded comments

* Added new logic (suggested in https://github.com/starship/starship/pull/5196#issuecomment-1566228913)

The new `detect_env_vars` now requires either SSH_ONLY to be false or the
environment variable SSH_CONNECTION to be set, so that is will be used

* Fixed typo

* Refactored the detect_env_vars function for early returns and better readability

* Change boolean logic for better readability

Co-authored-by: David Knaack <davidkna@users.noreply.github.com>

* Apply suggestions from code review to `detect_env_vars` method.

Co-authored-by: David Knaack <davidkna@users.noreply.github.com>

* Fixed bracket error & updated comments

- fixed bracket error in hostname.rs, after changes
- updated comments for context.rs, for the suggested changes

* Removed obsolete warning from docs/config/README.md

Co-authored-by: David Knaack <davidkna@users.noreply.github.com>

---------

Co-authored-by: Dominik Sander <dsander@users.noreply.github.com>
Co-authored-by: David Knaack <davidkna@users.noreply.github.com>

.github/config-schema.json

@@ -798,6 +798,7 @@
     },
     "hostname": {
       "default": {
+        "detect_env_vars": [],
         "disabled": false,
         "format": "[$ssh_symbol$hostname]($style) in ",
         "ssh_only": true,
@@ -3740,6 +3741,13 @@
           "default": ".",
           "type": "string"
         },
+        "detect_env_vars": {
+          "default": [],
+          "type": "array",
+          "items": {
+            "type": "string"
+          }
+        },
         "format": {
           "default": "[$ssh_symbol$hostname]($style) in ",
           "type": "string"