Immediately issue auto-detection when changing block mode to Auto-detect

[steobrien]

If I type the following, it’s auto-detected as Plain Text, not Markdown:

# This is some example markdown

## Subheading

I would expect it to be recognized:
- a list
- goes here
- and here

## Another

Or *how* about some _inline_ formatting? It doesn’t seem to do it.

Manually running auto-detect doesn't seem to fix it.

However, if I paste it fresh into a new block, it is correctly detected as Markdown.

[heyman]

The problem here is that selecting Auto-detect doesn't actually trigger an auto-detection immediately. It turns on auto-detection for the block (by changing its mode to Plain Text (auto)), but the auto-detection won't be triggered until a significant change to the content has been made.

I think it's okay that selecting Auto-detect changes the block mode to Plain Text (auto) (because you wouldn't select auto-detect if you were happy with the current mode, and plain text is a good default fallback), but when Auto-detect is selected, Heynote should immediately issue an auto-detection.