"export" was a named node despite always having fixed spelling, with
no variation to expose to queries. Anonymous string literals are
already fully queryable ("export" @keyword), so the named production
added a redundant node in the tree for no benefit.
Keeps the alias("export", $.identifier) in the key field, since that
usage is a genuine reclassification (export as an identifier, not the
keyword) and still needs to be named.
Updated corpus tests to use :cst format where "export" and "="
presence needs to be asserted explicitly.
