Create a struct with all of Hugo's config options

Primary motivation is documentation, but it will also hopefully simplify the code.

Also,

* Lower case the default output format names; this is in line with the custom ones (map keys) and how
it's treated all the places. This avoids doing `stringds.EqualFold` everywhere.

Closes #10896
Closes #10620

testscripts/commands/mod_npm.txt

@@ -0,0 +1,23 @@
+# Test mod npm.
+
+hugo mod npm pack
+cmp package.hugo.json golden/package.hugo.json
+
+-- hugo.toml --
+baseURL = "https://example.org/"
+-- package.json --
+{
+  "name": "test",
+  "version": "1.0.0",
+  "dependencies": {
+    "mod": "foo-bar"
+  }
+}
+-- golden/package.hugo.json --
+{
+  "name": "test",
+  "version": "1.0.0",
+  "dependencies": {
+    "mod": "foo-bar"
+  }
+}