yml: don’t crash when (trying to) merge anchors that aren’t dictionaries

Up until now, we only asserted the value being merged in was a
dictionary.

Now we do a proper check and return a real error message instead.

Closes #32

CHANGELOG.md

@@ -13,6 +13,11 @@
 ### Deprecated
 ### Removed
 ### Fixed
+
+* Crash when merging non-dictionary anchors in the YAML configuration
+  (https://codeberg.org/dnkl/yambar/issues/32).
+
+
 ### Security
 ### Contributors