Implement [[ fragment literals ]] (#1614)

This feature is referred to as "code/data literals" in ASMotor,
and simply as "literals" in some older assemblers like MIDAS
for the PDP-10. RGBASM already had the "section fragments"
feature for keeping disparate contents together when linked,
so these worked naturally as "fragment literals".

src/util.cpp

@@ -7,7 +7,8 @@
 #include <stdio.h>
 
 bool startsIdentifier(int c) {
-	// This returns false for anonymous labels, which internally start with a '!'
+	// This returns false for anonymous labels, which internally start with a '!',
+	// and for section fragment literal labels, which internally start with a '$'.
 	return (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z') || c == '.' || c == '_';
 }