[mod] revision of the settings_loader

The intention of this PR is to modernize the settings_loader implementations.
The concept is old (remember, this is partly from 2014), back then we only had
one config file, meanwhile we have had a folder with config files for a very
long time.  Callers can now load a YAML configuration from this folder as
follows ::

    settings_loader.get_yaml_cfg('my-config.yml')

- BTW this is a fix of #3557.

- Further the `existing_filename_or_none` construct dates back to times when
  there was not yet a `pathlib.Path` in all Python versions we supported in the
  past.

- Typehints have been added wherever appropriate

At the same time, this patch should also be downward compatible and not
introduce a new environment variable. The localization of the folder with the
configurations is further based on:

    SEARXNG_SETTINGS_PATH (wich defaults to /etc/searxng/settings.yml)

Which means, the default config folder is `/etc/searxng/`.

ATTENTION: intended functional changes!

 If SEARXNG_SETTINGS_PATH was set and pointed to a not existing file, the
 previous implementation silently loaded the default configuration.  This
 behavior has been changed: if the file or folder does not exist, an
 EnvironmentError exception will be thrown in future.

Closes: https://github.com/searxng/searxng/issues/3557
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

docs/admin/settings/index.rst

@@ -1,3 +1,5 @@
+.. _searxng settings.yml:
+
 ========
 Settings
 ========