Add missing (e)TeX primitives

- Add Knuth TeX primitives `\csname` and `\endcsname`
- Add eTeX primitives `\unless`, `\ifcsname`, `\ifdefined`, and `\iffontchar`

Syntaxes/TeX.plist

@@ -23,7 +23,7 @@
 				</dict>
 			</dict>
 			<key>match</key>
-			<string>(\\)(backmatter|else|fi|frontmatter|mainmatter|if(case|cat|dim|eof|false|hbox|hmode|inner|mmode|num|odd|true|vbox|vmode|void|x)?)\b</string>
+			<string>(\\)(backmatter|csname|else|endcsname|fi|frontmatter|mainmatter|unless|if(case|cat|csname|defined|dim|eof|false|fontchar|hbox|hmode|inner|mmode|num|odd|true|vbox|vmode|void|x)?)\b</string>
 			<key>name</key>
 			<string>keyword.control.tex</string>
 		</dict>