This fixes a regression introduced in v0.93.0 where previously, a
mixed-case key for lineNos would be successfully parsed. This change
moves the configuration key lowercasing step into the configuration
normalization stage, which is called whether the highlighting config
is being parsed from a `string` or a `map`.
Fixes#10682
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
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
