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
Avoid creating a local copy of the highlight configuration when no
options are passed.
Benchmarks of building the docs site:
name old time/op new time/op delta
DocsSite-2 1.94s ± 4% 1.93s ± 4% ~ (p=0.841 n=5+5)
name old alloc/op new alloc/op delta
DocsSite-2 666MB ± 1% 656MB ± 0% -1.48% (p=0.008 n=5+5)
name old allocs/op new allocs/op delta
DocsSite-2 8.85M ± 0% 8.76M ± 0% -1.04% (p=0.029 n=4+4)
This commit adds the fast and CommonMark compliant Goldmark as the new default markdown handler in Hugo.
If you want to continue using BlackFriday as the default for md/markdown extensions, you can use this configuration:
```toml
[markup]
defaultMarkdownHandler="blackfriday"
```
Fixes#5963Fixes#1778Fixes#6355
