Fix negative double parsing in 'json::readDouble' (#1359)

Fix #1355: `-1.5` gets parsed as `-1.5` instead of `-0.5`
Replaces #1356: I made a new PR so that we can add tests

---------

Co-authored-by: Joshua Schlucke <joshua.dev.no-reply@ochre.ovh>

Author: jiribenes

examples/stdlib/json.check

@@ -2,6 +2,8 @@
 
 a
 
+-1.5
+
 a
 b
 f

libraries/common/json.effekt

@@ -109,7 +109,7 @@ def readDouble(): Double / { Scan[Char], Exception[WrongFormat]} = {
   }
   def fraction(pre: Int): Double / fail = {
     if (succeeds { must { readIf('.') } }) {
-      var b = 0.1
+      var b = if (pre >= 0) 0.1 else -0.1
       var r = pre.toDouble
       while (optionally[Int] { must { readDigit() } } is Some(d)) {
         r = r + b * d.toDouble
@@ -316,6 +316,14 @@ namespace test {
 
     println("")
 
+    feed("-1.5") {
+      with scanner[Char]
+      with on[WrongFormat].report
+      println(readDouble())
+    }
+
+    println("")
+
     // Parse example
     feed("""{ "a": null, "b": [true,false,false,true], "f": 12.532   }"""){
       with scanner[Char]