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

minifiers/config.go

@@ -99,10 +99,10 @@ func decodeConfig(cfg config.Provider) (conf minifyConfig, err error) {
 
 	// Handle upstream renames.
 	if td, found := m["tdewolff"]; found {
-		tdm := cast.ToStringMap(td)
+		tdm := maps.ToStringMap(td)
 		for _, key := range []string{"css", "svg"} {
 			if v, found := tdm[key]; found {
-				vm := cast.ToStringMap(v)
+				vm := maps.ToStringMap(v)
 				if vv, found := vm["decimal"]; found {
 					vvi := cast.ToInt(vv)
 					if vvi > 0 {