refactor: simplify icon canvas updates

[MichaHuhn]

Simplify the icon canvas updates in IconCanvas.vue.

I'm using watchEffect() here. It works similar to watch() but you don't need to pass in the source data you want to watch.

The triggerGenerator() function will be executed only if the icon canvas is initialized and it will be executed whenever the currentIcon in the iconsStore updates.

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It's a good practice in Vue to put normal TypeScript functions into normal TypeScript files, so Vue components only need to manage Vue related code.
The awesome side effect is that you can unit-test those TypeScript functions very easily without needing to mount the Vue components.

That's why I moved the triggerGenerator() function into the generators.ts file next to the getMatchingGenerator() function.
The component now only needs to call this function.