Create page variable for merged page and site params like

Currently, we can do .Param "" to get the page param, or, if that doesn’t exist, then the site param, if it exists, or nil otherwise.

If we want to get all the “merged” parameters nested in the same place, we have to do it individually:

$bar := .Param ""
$baz := .Param "foo.baz"

It would be useful to be able to be able to collect all the merged parameters nested under the same parameter “path”:

$foo := .Merged "foo"
$bar := $
$baz := $foo.baz

It would be even more useful if we could use normal map accessor syntax:

$foo :=
$bar := $
$baz := $foo.baz

$bez := .Merged.bez

In my case, this would be useful when using a partial template that accepts the same config map from two different places in the config tree, e.g. article (containing title_font_size) and article_list (containing title_font_size).