Fix scope_depth to return 0 for root scope

[mc1098]

Hi 👋,

Just a quick fix so that the scope_depth function returns 0 for the root scope as stated by the comment -
I also added some tests for the function too.

This function is only ever used in this repository for the following SSR test:

sycamore/packages/sycamore/tests/ssr/main.rs

Lines 120 to 136 in 487877f

	#[test]
	fn using_cx_in_dyn_node_creates_nested_scope() {
	let _ = sycamore::render_to_string(|cx| {
	let outer_depth = scope_depth(cx);
	let inner_depth = create_ref(cx, Cell::new(0));
	let node = view! { cx,
	p {
	({
	inner_depth.set(scope_depth(cx));
	view! { cx, }
	})
	}
	};
	assert_eq!(inner_depth.get(), outer_depth + 1);
	node
	});
	}

that relies solely on the fact that depth is incremented on child scopes.

[codecov]

Codecov Report

Merging #424 (f11d992) into master (487877f) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #424      +/-   ##
==========================================
+ Coverage   64.87%   64.96%   +0.09%     
==========================================
  Files          52       52              
  Lines        8256     8278      +22     
==========================================
+ Hits         5356     5378      +22     
  Misses       2900     2900              

Impacted Files	Coverage Δ
packages/sycamore-reactive/src/context.rs	100.00% <100.00%> (ø)

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[lukechu10]

I left a few comments. Also thanks for adding tests!

[lukechu10]

Although this is a scoped function, I believe it technically still needs to be unsafe. In that case, it might just be better to inline unsafe { &*x }

[lukechu10]

I have another idea of an implementation that might be simpler:

while let Some(next) = current.parent.map(unsafe { &*x }) { ... }