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 #5963
Fixes #1778
Fixes #6355
This commmit prepares for the addition of Goldmark as the new Markdown renderer in Hugo.
This introduces a new `markup` package with some common interfaces and each implementation in its own package.
See #5963
