Disallow minus sign before symbol name in bracketed macro arguments (#2015)

This was being silently allowed but without actually negating the symbol's value

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

@@ -0,0 +1,5 @@
+MACRO test
+	println \<2>, " vs ", \<-2>
+	println \<_NARG>, " vs ", \<-_NARG>
+ENDM
+	test "hello", "goodbye"

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

@@ -0,0 +1,5 @@
+error: No digit after minus sign in bracketed macro argument
+    at macro-arg-negative-symbol.asm::test(3) <- macro-arg-negative-symbol.asm(5)
+error: syntax error, unexpected end of line
+    at macro-arg-negative-symbol.asm::test(3) <- macro-arg-negative-symbol.asm(5)
+Assembly aborted with 2 errors

test/asm/macro-arg-negative-symbol.out

@@ -0,0 +1,2 @@
+goodbye vs hello
+goodbye vs