switch to old tab if active is closed

[forivall]

Fixes #36 (scenario 2)

I also add a bunch of jsdoc and such so typescript can understand the code. (also, changed substr to slice, since substr is deprecated). Could use substring instead, but i prefer slice). If undesired, we can just merge the commit with the change: https://github.com/brcontainer/prevent-duplicate-tabs/compare/main...forivall:prevent-duplicate-tabs:feature/switch-to-existing