This commit adds support for using the `cascade` keyword in your configuration file(s), e.g. `config.toml`.
Note that
* Every feature of `cascade` is available, e.g. `_target` to target specific page sets.
* Pages, e.g. the home page, can overwrite the cascade defined in config.
Fixes#8741
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#8633Fixes#8618Fixes#8630
Updates #8591Closes#6680Closes#5192
