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

test/asm/fixed-point-syntax.err

@@ -0,0 +1,15 @@
+error: Invalid fixed-point constant, no significant digits after 'q'
+    at fixed-point-syntax.asm(10)
+error: Invalid fixed-point constant, no significant digits after 'q'
+    at fixed-point-syntax.asm(11)
+error: syntax error, unexpected symbol
+    at fixed-point-syntax.asm(11)
+error: syntax error, unexpected symbol
+    at fixed-point-syntax.asm(12)
+error: Invalid integer constant, trailing '_'
+    at fixed-point-syntax.asm(13)
+error: Invalid fixed-point constant, '_' after '.'
+    at fixed-point-syntax.asm(14)
+error: Invalid fixed-point constant, no significant digits after 'q'
+    at fixed-point-syntax.asm(15)
+Assembly aborted with 7 errors

test/asm/fixed-point-syntax.out

@@ -0,0 +1,10 @@
+$323D7
+$2A0000
+$4D2915B
+$18B
+$62B85
+$4
+$C570A
+$13333
+$13333
+$13333

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

test/asm/invalid-underscore.err

@@ -8,9 +8,9 @@ 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 '_'
+error: Invalid fixed-point constant, '_' after another '_'
     at invalid-underscore.asm(19)
-error: Invalid integer constant, '_' after another '_'
+error: Invalid fixed-point constant, '_' after another '_'
     at invalid-underscore.asm(20)
 error: Invalid integer constant, trailing '_'
     at invalid-underscore.asm(23)
@@ -26,6 +26,10 @@ error: Invalid fixed-point constant, trailing '_'
     at invalid-underscore.asm(28)
 error: Invalid integer constant, trailing '_'
     at invalid-underscore.asm(31)
-error: Invalid integer constant, '_' after another '_'
+error: Invalid fixed-point constant, '_' after '.'
     at invalid-underscore.asm(32)
-Assembly aborted with 15 errors
+error: syntax error, unexpected symbol
+    at invalid-underscore.asm(35)
+error: syntax error, unexpected symbol
+    at invalid-underscore.asm(36)
+Assembly aborted with 17 errors