Ensure there is always only a single instance of an Effekt Runner

[jiribenes]

Sometimes I see that I have like three "Effekt Runner" terminals open. No idea how thay happens, my hypothesis is that VSCode persists them across projects / exits and the plugin just makes a new one on start. Instead, we should perhaps look if there's a terminal called "Effekt Runner" and use it instead?

[jiribenes]

Hypothesis: this happens when we reload the extension. Perhaps we should also search for a terminals called "Effekt Runner", close em all but the recent one?