One at-rule, read back whole.
IgniterCss.analyze/2 reports only that an at-rule of some name exists. This
reports which one and how it was configured — the difference between knowing a
@plugin is present and knowing it was loaded as
@plugin "daisyui" { prefix: "d-"; }, which is what an installer needs before
it can generate code that agrees with the user's setup.
declarations is empty for an at-rule with no block, or one whose block holds
rules rather than declarations.