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

modules/collect.go

@@ -424,7 +424,7 @@ func (c *collector) applyThemeConfig(tc *moduleAdapter) error {
 		if err != nil {
 			c.logger.Warnf("Failed to read module config for %q in %q: %s", tc.Path(), themeTOML, err)
 		} else {
-			maps.ToLower(themeCfg)
+			maps.PrepareParams(themeCfg)
 		}
 	}