Fix it so disableKinds etc. does not get merged in from theme

Unless the merge strategy is set up to do so.

For `disableKinds` the current workaround is to make sure the project config has an entry, even if is empty:

```
disableKinds = []
```

Note that this issue only touches root, non-map config-values that either is not set in project config or in Hugo's defaults.

Fixes #8866

common/maps/params.go

@@ -76,6 +76,14 @@ func (p Params) Merge(pp Params) {
 	p.merge("", pp)
 }
 
+// MergeRoot transfers values from pp to p for new keys where p is the
+// root of the tree.
+// This is done recursively.
+func (p Params) MergeRoot(pp Params) {
+	ms, _ := p.GetMergeStrategy()
+	p.merge(ms, pp)
+}
+
 func (p Params) merge(ps ParamsMergeStrategy, pp Params) {
 	ns, found := p.GetMergeStrategy()