fix(svelt:events): ensure m prop is synced

[pedroyremolo]

Description

After solving the tutorial exercise of section Part 1. BasicSvelt > Events > DOM Events as requested by the tutorial, the pointer position was not being updated at the DOM. This was happening because the m object was not being assigned after changes, this PR solved it.

Disclaim: Opted for m = m; instead of whole object assignment because the next section already suggests it

[vercel]

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[Conduitry]

This isn't necessary. Assigning to m.x or m.y already invalidates m. The text updates when you just add the on:pointermove={handleMove}. What are you seeing?