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/fixed-point-syntax.asm

@@ -0,0 +1,15 @@
+; good
+println 3.14
+println 42.
+println 12_34.56_78
+println 12.34_q.5
+println 1_2.3_4_q15
+println 1.q2
+
+; bad
+println 12.34q0
+println 12.34q_15
+println 12.34q1_5
+println 1_.2
+println 1._2
+println 1.2q