Add a way to merge pages by language

As an example:

```html
{{ $pages := .Site.RegularPages | lang.Merge $frSite.RegularPages | lang.Merge $enSite.RegularPages }}
```

Will "fill in the gaps" in the current site with, from left to right, content from the French site, and lastly the English.

Fixes #4463

hugolib/shortcode_test.go

@@ -673,7 +673,8 @@ NotFound: {{< thisDoesNotExist >}}
 	writeSource(t, fs, "content/sect/mycsvpage.md", fmt.Sprintf(pageTemplateCSVOnly, "Single CSV"))
 	writeSource(t, fs, "content/sect/notfound.md", fmt.Sprintf(pageTemplateShortcodeNotFound, "Single CSV"))
 
-	require.NoError(t, h.Build(BuildCfg{}))
+	err := h.Build(BuildCfg{})
+	require.Equal(t, "logged 1 error(s)", err.Error())
 	require.Len(t, h.Sites, 1)
 
 	s := h.Sites[0]