CSS-Extras: Added even & odd keywords to n-th pattern (#1872)

`even` and `odd` are keywords which are also an `n-th` value. This adds it to the pattern which previously only captured values of the form `an+b`.

components/prism-css-extras.js

@@ -36,14 +36,20 @@ Prism.languages.css.selector = {
 				'operator': /[|~*^$]?=/
 			}
 		},
-		'n-th': {
-			pattern: /(\(\s*)[+-]?\d*[\dn](?:\s*[+-]\s*\d+)?(?=\s*\))/,
-			lookbehind: true,
-			inside: {
-				'number': /[\dn]+/,
-				'operator': /[+-]/
+		'n-th': [
+			{
+				pattern: /(\(\s*)[+-]?\d*[\dn](?:\s*[+-]\s*\d+)?(?=\s*\))/,
+				lookbehind: true,
+				inside: {
+					'number': /[\dn]+/,
+					'operator': /[+-]/
+				}
+			},
+			{
+				pattern: /(\(\s*)(?:even|odd)(?=\s*\))/i,
+				lookbehind: true
 			}
-		},
+		],
 		'punctuation': /[()]/
 	}
 };

tests/languages/css!+css-extras/selector_n-th_feature.test

@@ -2,6 +2,8 @@
 :nth-child(+2n - 1) {}
 :nth-child(2n) {}
 :nth-child(+5) {}
+:nth-child(even) {}
+:nth-child(odd) {}
 
 ----------------------------------------------------
 
@@ -54,6 +56,24 @@
 		["punctuation", ")"]
 	]],
 	["punctuation", "{"],
+	["punctuation", "}"],
+
+	["selector", [
+		["pseudo-class", ":nth-child"],
+		["punctuation", "("],
+		["n-th", "even"],
+		["punctuation", ")"]
+	]],
+	["punctuation", "{"],
+	["punctuation", "}"],
+
+	["selector", [
+		["pseudo-class", ":nth-child"],
+		["punctuation", "("],
+		["n-th", "odd"],
+		["punctuation", ")"]
+	]],
+	["punctuation", "{"],
 	["punctuation", "}"]
 ]