[lex.key] Add a proper grammar production for 'keyword'.

Move export-keyword, import-keyword, and module-keyword out of our table
of keyword identifiers (where they don't fit) and make them directly be
grammar productions for keyword instead.

Author: zygoloid

source/lex.tex

@@ -752,7 +752,14 @@
 
 \rSec1[lex.key]{Keywords}
 
-\enlargethispage{\baselineskip}%
+\begin{bnf}
+\nontermdef{keyword}\br
+    \textnormal{any identifier listed in \tref{lex.key}}\br
+    \grammarterm{import-keyword}\br
+    \grammarterm{module-keyword}\br
+    \grammarterm{export-keyword}
+\end{bnf}
+
 \pnum
 \indextext{keyword|(}%
 The identifiers shown in \tref{lex.key} are reserved for use
@@ -799,29 +806,26 @@
 \keyword{enum} \\
 \keyword{explicit} \\
 \keyword{export} \\
-\grammarterm{export-keyword} \\
-\columnbreak
 \keyword{extern} \\
+\columnbreak
 \keyword{false} \\
 \keyword{float} \\
 \keyword{for} \\
 \keyword{friend} \\
 \keyword{goto} \\
 \keyword{if} \\
-\grammarterm{import-keyword} \\
 \keyword{inline} \\
 \keyword{int} \\
 \keyword{long} \\
-\grammarterm{module-keyword} \\
 \keyword{mutable} \\
 \keyword{namespace} \\
 \keyword{new} \\
 \keyword{noexcept} \\
 \keyword{nullptr} \\
-\columnbreak
 \keyword{operator} \\
 \keyword{private} \\
 \keyword{protected} \\
+\columnbreak
 \keyword{public} \\
 \keyword{register} \\
 \keyword{reinterpret_cast} \\
@@ -836,10 +840,10 @@
 \keyword{struct} \\
 \keyword{switch} \\
 \keyword{template} \\
-\columnbreak
 \keyword{this} \\
 \keyword{thread_local} \\
 \keyword{throw} \\
+\columnbreak
 \keyword{true} \\
 \keyword{try} \\
 \keyword{typedef} \\