Fix some fixed-point literal lexing issues (#1914)

- No fractional digits are necessary after the decimal point,
  e.g. `42.` is valid instead of `42.0`
- Error messages refer to "fixed-point" not "integer" constants
- Test more carefully for lexing unrelated to underscores

test/asm/invalid-underscore.asm

@@ -30,3 +30,7 @@ println 123.456_
 ; bad ('_' next to '.')
 println 1_.618
 println 2._718
+
+; bad ('_' after 'q')
+println 1q_2
+println 1q2_3