feat: make export keyword anonymous.

"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.

grammar.js

@@ -24,17 +24,17 @@ module.exports = grammar({
       seq(
         field("key", choice(
           $.identifier,
-          alias($.export, $.identifier),
+          alias("export", $.identifier),
         )),
         "=",
         optional(field("value", $._value)),
         $._end_of_assignment,
       ),
       prec(1, seq(
-        $.export,
+        "export",
         field("key", choice(
           $.identifier,
-          alias($.export, $.identifier),
+          alias("export", $.identifier),
         )),
         "=",
         optional(field("value", $._value)),
@@ -43,7 +43,6 @@ module.exports = grammar({
     ),
 
     comment: _ => /\#[^\n]*/,
-    export: _ => token(prec(1, 'export')),
 
     identifier: _ => /[A-Za-z_][A-Za-z0-9_]*/,
     variable: $ => seq('$', choice(