fix(#584): Add support to yaml extension

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

testdata/includes_yaml/.gitignore

@@ -0,0 +1 @@
+*.txt

testdata/includes_yaml/Custom.ext

@@ -0,0 +1,16 @@
+version: '3'
+
+includes:
+  included: ./included
+  custom: ./included/custom.yaml
+
+tasks:
+  default:
+    cmds:
+      - task: gen
+      - task: included:gen
+      - task: custom:gen
+
+  gen:
+    cmds:
+      - echo main > main.txt

testdata/includes_yaml/included/Taskfile.yaml

@@ -0,0 +1,6 @@
+version: '3'
+
+tasks:
+  gen:
+    cmds:
+      - echo included_with_yaml_extension > included_with_yaml_extension.txt

testdata/includes_yaml/included/custom.yaml

@@ -0,0 +1,6 @@
+version: '3'
+
+tasks:
+  gen:
+    cmds:
+      - echo included_with_custom_file > included_with_custom_file.txt