✨ Grammar: Highlight ABNF core rules

The grammar now adds the scope `support.constant` to the core rules,
specified in Appendix B of the [ABNF RFC][].

[ABNF RFC]: https://tools.ietf.org/pdf/rfc5234.pdf

Syntaxes/ABNF.tmLanguage

@@ -548,6 +548,35 @@
 				</dict>
 			</array>
 		</dict>
+		<key>rule-name-core</key>
+		<dict>
+			<key>captures</key>
+			<dict>
+				<key>rule</key>
+				<dict>
+					<key>name</key>
+					<string>support.constant.core-rule.$1.abnf</string>
+				</dict>
+			</dict>
+			<key>match</key>
+			<string>(?x)\b(?&lt;rule&gt;
+                      ALPHA     |
+                      BIT       |
+                      CHAR      |
+                      CR(?:LF)? | 
+                      CTL       |
+                      DIGIT     |
+                      DQUOTE    |
+                      HEXDIG    |
+                      HTAB      |
+                      LF        |
+                      LWSP      |
+                      OCTECT    |
+                      SP        |
+                      VCHAR     |
+                      WSP       
+                      )\b</string>
+		</dict>
 		<key>rule-name-plain</key>
 		<dict>
 			<key>captures</key>
@@ -623,6 +652,10 @@
 					<key>include</key>
 					<string>#string-double-quoted</string>
 				</dict>
+				<dict>
+					<key>include</key>
+					<string>#rule-name-core</string>
+				</dict>
 				<dict>
 					<key>include</key>
 					<string>#rule-name</string>

Tests/Example.abnf

@@ -1,7 +1,7 @@
 ; In ABNF comments start with a single semicolon.
 
 ; An ABNF rule can contain multiple sub-rules.
-rule = subrule1 subrule2 subrule3
+rule = WSP subrule2 DIGIT
 
 ; We can also use optional angle brackets around a rule name.
 <oh-look-ma-angle-brackets> = "[]"; To specify a sequence of characters we use