fix(canvas): Change default stroke line_width from 2.0 to 1.0

[jtvhd6]

Fixes #4381

The HTML5 Canvas specification defines lineWidth = 1.0 as the default.
Toga's Canvas API is intended to mirror the HTML5 Canvas API, so the
default line_width should match.

Changed the default from 2.0 to 1.0 in:

• core/src/toga/widgets/canvas/canvas.py
• cocoa/src/toga_cocoa/widgets/canvas.py
• iOS/src/toga_iOS/widgets/canvas.py

[HalfWhitt]

Hi @jtvhd6, thanks for the contribution. There are some issues here. First of all, there's already a PR (#4396) in flight for this issue. There's no sense in duplicating work. Granted, that PR hasn't seen activity in a couple weeks, but it's always prudent (and polite) to drop a comment on such a PR checking in with the original author to see if they're still around and working on it, before forging ahead.

Beyond that, you can see that the towncrier check is failing; this is because this PR lacks a change note, as described in our contribution guide.

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[freakboy3742]

Closing due to lack of response to feedback.