Support numeric symbol names in \(parentheses)

For example, \(_NARG) will get the last argument

test/asm/invalid-macro-arg-symbol.asm

@@ -0,0 +1 @@
+\(foo)

test/asm/invalid-macro-arg-symbol.err

@@ -0,0 +1,2 @@
+FATAL: invalid-macro-arg-symbol.asm(1):
+    Parenthetic symbol "foo" does not exist

test/asm/parenthetic-macro-args.asm

@@ -12,6 +12,8 @@ MACRO mac
 	println \(2__) + \(1_2) + \(\1)
 x = 2
 	println \({d:x}) + \(1_{d:x}) + \(\(\(13)))
+y equs "NARG"
+	println \(x) + \(1_{d:x}_) + \(\(\(_{y})))
 ENDM
 
 	mac 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 1

test/asm/parenthetic-macro-args.out

@@ -4,3 +4,4 @@ next = C
 last = D
 $F0
 $F0
+$F0