fix(#584): Add support to yaml extension

- init creates Taskfile.yaml
- add changelog entry
- add zsh completion support for Taskfile.yaml

testdata/ignore_nil_elements/includes/Taskfile.yml

@@ -0,0 +1,9 @@
+version: '3'
+
+includes:
+  inc: inc.yml
+
+tasks:
+  default:
+    cmds:
+      - task: inc:default

testdata/ignore_nil_elements/includes/inc.yml

@@ -0,0 +1,7 @@
+version: '3'
+
+tasks:
+  default:
+    cmds:
+      - 
+      - echo "string-slice-1"