tpl: Allow the partial template func to return any type

This commit adds support for return values in partials.

This means that you can now do this and similar:

    {{ $v := add . 42 }}
    {{ return $v }}

Partials without a `return` statement will be rendered as before.

This works for both `partial` and `partialCached`.

Fixes #5783

tpl/partials/init.go

@@ -36,6 +36,13 @@ func init() {
 			},
 		)
 
+		// TODO(bep) we need the return to be a valid identifier, but
+		// should consider another way of adding it.
+		ns.AddMethodMapping(func() string { return "" },
+			[]string{"return"},
+			[][2]string{},
+		)
+
 		ns.AddMethodMapping(ctx.IncludeCached,
 			[]string{"partialCached"},
 			[][2]string{},