Require underscores to actually be digit separators (#1812)

Multiple, trailing, or next to decimal point are errors

test/asm/bracketed-macro-args.asm

@@ -9,11 +9,11 @@ ENDM
 	printargs A, B, C, D
 
 MACRO mac
-	println \<2__> + \<1_2> + \<\1>
+	println \<2> + \<1_2> + \<\1>
 	def x = 2
 	println \<{d:x}> + \<1_{d:x}> + \<\<\<13>>>
 	def y equs "NARG"
-	println \<x> + \<1_{d:x}_> + \<\<\<_{y}>>>
+	println \<x> + \<1_{d:x}> + \<\<\<_{y}>>>
 ENDM
 
 	mac 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 1

test/asm/invalid-numbers.err

@@ -2,11 +2,11 @@ error: Invalid integer constant, no digits after '$'
     at invalid-numbers.asm::try(2) <- invalid-numbers.asm(11)
 error: Invalid graphics constant, no digits after '`'
     at invalid-numbers.asm::try(2) <- invalid-numbers.asm(12)
-error: Invalid integer constant, no digits after '%'
+error: Invalid integer constant, no digits after "0b"
     at invalid-numbers.asm::try(2) <- invalid-numbers.asm(13)
-error: Invalid integer constant, no digits after '&'
+error: Invalid integer constant, no digits after "0o"
     at invalid-numbers.asm::try(2) <- invalid-numbers.asm(14)
-error: Invalid integer constant, no digits after '$'
+error: Invalid integer constant, no digits after "0x"
     at invalid-numbers.asm::try(2) <- invalid-numbers.asm(15)
 warning: Integer constant is too large [-Wlarge-constant]
     at invalid-numbers.asm::try(2) <- invalid-numbers.asm(18)

test/asm/invalid-underscore.asm

@@ -0,0 +1,31 @@
+; good
+println 123_456
+println %_1010_1010
+println 0b_1010_1010
+println &_555_555
+println 0o_777_777
+println $_dead_beef
+println 0x_cafe_babe
+println `_0101_2323
+println 12_34.56_78
+println 12_34.56_q8
+
+; bad (multiple '_')
+println 123__456
+println %1010__1010
+println &123__456
+println $abc__def
+println `0101__2323
+println 3.14__15
+println 2.718__Q16
+
+; bad (trailing '_')
+println 12345_
+println 0b101010_
+println 0o123456_
+println 0xabcdef_
+println `01230123_
+
+; bad ('_' next to '.')
+println 1_.618
+println 2._718

test/asm/invalid-underscore.err

@@ -0,0 +1,29 @@
+error: Invalid integer constant, '_' after another '_'
+    at invalid-underscore.asm(14)
+error: Invalid integer constant, '_' after another '_'
+    at invalid-underscore.asm(15)
+error: Invalid integer constant, '_' after another '_'
+    at invalid-underscore.asm(16)
+error: Invalid integer constant, '_' after another '_'
+    at invalid-underscore.asm(17)
+error: Invalid integer constant, '_' after another '_'
+    at invalid-underscore.asm(18)
+error: Invalid integer constant, '_' after another '_'
+    at invalid-underscore.asm(19)
+error: Invalid integer constant, '_' after another '_'
+    at invalid-underscore.asm(20)
+error: Invalid integer constant, trailing '_'
+    at invalid-underscore.asm(23)
+error: Invalid integer constant, trailing '_'
+    at invalid-underscore.asm(24)
+error: Invalid integer constant, trailing '_'
+    at invalid-underscore.asm(25)
+error: Invalid integer constant, trailing '_'
+    at invalid-underscore.asm(26)
+error: Invalid graphics constant, trailing '_'
+    at invalid-underscore.asm(27)
+error: Invalid integer constant, trailing '_'
+    at invalid-underscore.asm(30)
+error: Invalid integer constant, '_' after another '_'
+    at invalid-underscore.asm(31)
+Assembly aborted with 14 errors!

test/asm/invalid-underscore.out

@@ -0,0 +1,24 @@
+$1E240
+$AA
+$AA
+$2DB6D
+$3FFFF
+$DEADBEEF
+$CAFEBABE
+$F55
+$4D2915B
+$4D28F
+$1E240
+$AA
+$A72E
+$ABCDEF
+$F55
+$32439
+$2B7CF
+$3039
+$2A
+$A72E
+$ABCDEF
+$3355
+$19E35
+$2B7CF

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

@@ -14,15 +14,15 @@ _1234::
 
 ; with underscores
 	dw _1234 ; label
-	db 123, 1_23, 1__23 ; decimal
+	db 123, 1_23, 12_3 ; decimal
 	dw 12_345 ; decimal
 	dw $ab_cd ; hex
-	db &2_0_0_ ; octal
+	db &2_0_0 ; octal
 	db %1111_0000, %1_0 ; binary
-	dl 6_._283_185 ; fixed point
-	dw `0123_3210, `00_33_22_11_ ; gfx
+	dl 6.283_185 ; fixed point
+	dw `0123_3210, `_00_33_22_11 ; gfx
 
 ; underscores with custom digits
 	opt g.ABC, b.X
-	db %.X.X_..XX_
-	dw `.A.B_.C.._
+	db %.X.X_..XX
+	dw `.A.B_.C..