chore: tweak template generation logic

[Rich-Harris]

#13075 added support for expressions in templates, but I think we can neaten it up a bit — rather than creating new push_quasi and push_expression functions, state.template can just be an Array<string | Expression> that gets processed later. This is more efficient, and in line with how similar problems are solved elsewhere in the codebase. (In terms of variable naming: it's awkward to have an array with a singular name — quasi — which then forces the individual quasi to be called something cumbersome like quasi_to_add. It should just be quasis.push(quasi)! Though really it should have been strings.push(string), because we're not actually dealing with quasis at this point.)

Another benefit of this approach is that we can vary the output more easily — we could, if we chose, emit "a" + b + "c" instead of `a${b}c` more easily than if we're constructing the expression on the fly.

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[trueadm]

Definitely more consistent with other parts of the codebase.