Applicable when autoHeadingIDType is either `github` or `github-ascii`.
When autoHeadingIDType is `blackfriday`, the existing code removes
trailing whitespace while iterating through the characters, using
a boolean "futureDash" mechanism.
Fixes#6798
You can turn off this behaviour:
```toml
[markup]
[markup.goldmark]
[markup.goldmark.parser]
autoHeadingIDAsciiOnly = true
```
Note that the `anchorize` now adapts its behaviour depending on the default Markdown handler.
Fixes#6616
