Add "geolocation task queue"

[marcoscaceres]

Closes #80

@annevk, I did as you suggested:

1. Collapsed the "Acquire a watch id" labelled steps.
2. "Run these steps [=in parallel=]:".
3. Tied all callback invocations to a microtask.
4. And now I return |watchId| as the final step.

As I had to re-indent the algorithm, probably best viewed in PR-Preview.

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Preview | Diff

[reillyeon]

LGTM with some nits.

[annevk]

Pretty sure that a microtask can only be queued in the context of a task. When you're in parallel you have to queue a task.

[marcoscaceres]

@annevk, ok, I was a bit unsure. I'll queue a task, but was unsure what task source to go with... should I mint a new one for geolocation? Or do you have a suggestion for an existing one?

[marcoscaceres]

Gave this some thought... as every browser has its own dedicated location service manager, I'm going to mint a "geolocation task source".

[marcoscaceres]

@annevk, added the new "geolocation task source" 🥫.

[annevk]

Seems reasonable. You want to use "queue a global task" and probably pass "this's relevant global object" (that you put in a variable before going in parallel).