Remove deprecated "$" and "^" prefixes

`$` was a variable prefix that make it being evaluated as shell. It was
replaced with `sh:`.

`^` is a command prefix that make it run another task. It was replaced
with `task:`.

These were added long ago when we were experimenting with stuff and kept for
some time for backward compatibility reasons, but sometimes causes confusion
and I think the time to remove the code came.

Closes #644
Closes #645
Ref #642

Co-authored-by: Trite <60318513+Trite8Q1@users.noreply.github.com>

testdata/vars/v2/Taskfile.yml

@@ -35,7 +35,8 @@ tasks:
       - echo '{{.TASK}}' > task_name.txt
     vars:
       FOO: foo
-      BAR: $echo bar
+      BAR:
+        sh: echo bar
       BAZ:
         sh: echo baz
       TMPL_FOO: "{{.FOO}}"

testdata/vars/v2/Taskvars.yml

@@ -1,5 +1,6 @@
 FOO2: foo2
-BAR2: $echo bar2
+BAR2:
+  sh: echo bar2
 BAZ2:
   sh: echo baz2
 TMPL2_FOO: "{{.FOO}}"