Implement Line tool

[edwin0cheng]

This PR implemented Line-Tool, which just default color of white for now, with some default thickness :

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This change is 

[Keavon]

Reviewed 7 of 7 files at r1.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @edwin0cheng)

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core/document/src/lib.rs, line 25 at r1 (raw file):

stroke="white"

Could you please change this from stroke="white" to style="stroke: #fff;" for consistency? It'll be changed later, but it's used by a JavaScript hack which replaces #fff with random colors.

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core/editor/src/tools/line.rs, line 54 at r1 (raw file):

				LineToolFsmState::Ready
			}

Nitpick: please delete this blank line between the two match arms. Thanks.

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core/editor/src/tools/line.rs, line 57 at r1 (raw file):

			// TODO - Check for left mouse button
			(LineToolFsmState::LmbDown, Event::MouseUp(mouse_state)) => {
				let r = data.drag_start.distance(&mouse_state.position);

Let's rename r (which I assume came from the circle radius) to distance (and avoid single-character variable names in favor of expressiveness over shortness).

[edwin0cheng]

Fixed !

[Keavon]

Reviewed 2 of 2 files at r2.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @edwin0cheng)