Require underscores to actually be digit separators (#1812)

Multiple, trailing, or next to decimal point are errors

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