`servings`/`yield` should allow floating-point numbers

[tomodachi94]

I have a recipe that has a yield of 1.75 cups, and I marked it as such in the metadata:

---
servings: 1.75 # cup
---

Some instructions...

However, the playground gives me a warning:

Warning: Unsupported value for key: 'servings'
  ╰▶  Expected 'number' but got 'number'
  ╭─[playground]
  │
2 │ servings: 1.75 # cup
  ┆ ─                   
──╯

And scaling doesn't work (behaves as if servings: 1 is set).

It looks like this is because the as_servings function in metadata.rs only tries u32 and string:

cooklang-rs/src/metadata.rs

Lines 393 to 397 in b86eec6

	fn as_servings(&self) -> Option<Servings> {
	// Try as number first
	if let Some(n) = self.as_u32() {
	return Some(Servings::Number(n));
	}

Workaround

I could change the unit to be a whole number, but this isn't ideal.