Tracking issue template: fine-grained information on style update status

Inspired by some of the communication issues around the stabilization of
`let`-chains, give more fine-grained information about the status of
updating style for any new syntax.

This does not change the process or blockers in any way; it only
*documents* the current state in the tracking issue. For instance, in
the case of `let`-chains, we would have checked the boxes for "Style
team decision" and "(non-blocking) Formatting has been implemented", and
not checked the box for the style guide. That would have then provided
better supporting information for any decisions.

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