Implement include optional=true (#3022)

* feat(niri): support `include optional=true "filename.kdl"`

* chore: warn if optional include ENOENT

* chore: validate include directive arguments and properties

Add proper validation to reject:
- Extra arguments beyond the path
- Unknown properties (other than "optional")
- Unexpected child nodes

* docs: implement suggested typographical/prose changes

* fixes

---------

Co-authored-by: Ivan Molodetskikh <yalterz@gmail.com>

docs/wiki/Configuration:-Include.md

@@ -114,6 +114,30 @@ window-rule {
 }
 ```
 
+### Optional includes
+
+<sup>Since: next release</sup>
+
+By default, including a nonexistent file will cause an error.
+You can allow nonexistent includes by setting `optional=true`:
+
+```kdl,must-fail
+// Won't fail if this file doesn't exist.
+include optional=true "optional-config.kdl"
+
+// Regular include, will fail if the file doesn't exist.
+include "required-config.kdl"
+```
+
+When an optional include file is missing, niri will emit a warning in the logs on every config reload.
+This reminds you that the file is missing while still loading the config successfully.
+
+The optional file is still watched for changes, so if you create it later, the config will automatically reload and apply the new settings.
+
+Note that `optional` only affects whether a missing file causes an error.
+If the file exists but contains invalid syntax or other errors, those errors will still cause a parsing failure.
+
+
 ### Merging
 
 Most config sections are merged between includes, meaning that you can set only a few properties, and only those properties will change.