Misc config loading fixes

The main motivation behind this is simplicity and correctnes, but the new small config library is also faster:

```
BenchmarkDefaultConfigProvider/Viper-16         	  252418	      4546 ns/op	    2720 B/op	      30 allocs/op
BenchmarkDefaultConfigProvider/Custom-16        	  450756	      2651 ns/op	    1008 B/op	       6 allocs/op
```

Fixes #8633
Fixes #8618
Fixes #8630
Updates #8591
Closes #6680
Closes #5192

markup/highlight/config_test.go

@@ -17,16 +17,15 @@ package highlight
 import (
 	"testing"
 
-	"github.com/spf13/viper"
-
 	qt "github.com/frankban/quicktest"
+	"github.com/gohugoio/hugo/config"
 )
 
 func TestConfig(t *testing.T) {
 	c := qt.New(t)
 
 	c.Run("applyLegacyConfig", func(c *qt.C) {
-		v := viper.New()
+		v := config.New()
 		v.Set("pygmentsStyle", "hugo")
 		v.Set("pygmentsUseClasses", false)
 		v.Set("pygmentsCodeFences", false)