Fix invalid character in bracketed macro argument

The invalid character should not be consumed by `shiftChar()`.

test/asm/character-escapes.out

@@ -3,5 +3,6 @@
  	)
 (
  
- 	)
-illegal character escape z?
+ 	)
+illegal character escape z?
+invalid character 

test/asm/empty-raw-identifier.out

@@ -1,2 +1,2 @@
 all args: a,#b,c,1,#2,3
-bad args: >, >
+bad args: ?>, >

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

@@ -0,0 +1,6 @@
+MACRO m
+	println "¿", """\<_NARG""", "?"
+	println """done!"""
+ENDM
+
+	m one, two, three

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

@@ -0,0 +1,3 @@
+error: Invalid character '"' in bracketed macro argument
+    at invalid-bracketed-macro-arg.asm::m(2) <- invalid-bracketed-macro-arg.asm(6)
+Assembly aborted with 1 error

test/asm/invalid-bracketed-macro-arg.out

@@ -0,0 +1,2 @@
+¿?
+done!

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

@@ -1,5 +1,5 @@
 error: Invalid character '!' in bracketed macro argument
     at invalid-macro-arg-character.asm(1)
-error: syntax error, unexpected >
+error: syntax error, unexpected !
     at invalid-macro-arg-character.asm(1)
 Assembly aborted with 2 errors