Allow underscores in numeric literals

Fixes #539

Changes \@'s output to start with "_u", not "_", so it will be valid within labels but not numerics

test/asm/underscore-in-numeric-literal.asm

@@ -0,0 +1,21 @@
+SECTION "Test", ROM0
+
+_1234::
+
+; without underscores
+	dw _1234 ; label
+	db 123, 123, 123 ; decimal
+	dw 12345 ; decimal
+	dw $abcd ; hex
+	db &200 ; octal
+	db %11110000, %10 ; binary
+	dl 6.283185 ; fixed point
+
+; with underscores
+	dw _1234 ; label
+	db 123, 1_23, 1__23 ; decimal
+	dw 12_345 ; decimal
+	dw $ab_cd ; hex
+	db &2_0_0_ ; octal
+	db %1111_0000, %1_0 ; binary
+	dl 6_._283_185 ; fixed point