If this attribute is set, the writer will output a label.
(This overrides labels based on identifiers, when identifiers
are present.)
Note that labels in typst need not be unique and may be used to
style elements using show rules. For example, one can attach
an `excerpt` label to all Divs that are excerpts, and style this
using a show rule like `#show <excerpt>: set text(blue)`.
See discussion at #11794.