Support positive sign in scientific notation (#706)

* Support positive sign in scientific notation
* Add exponent with positive sign to docs

doc/site/values.markdown

@@ -19,6 +19,7 @@ Like other scripting languages, Wren has a single numeric type:
 double-precision floating point. Number literals look like you expect coming
 from other languages:
 
+
 <pre class="snippet">
 0
 1234
@@ -26,6 +27,9 @@ from other languages:
 3.14159
 1.0
 -12.34
+0.0314159e02
+0.0314159e+02
+314.159e-02
 </pre>
 
 Numbers are instances of the [Num][] class.

src/vm/wren_compiler.c

@@ -762,8 +762,11 @@ static void readNumber(Parser* parser)
   // See if the number is in scientific notation.
   if (matchChar(parser, 'e') || matchChar(parser, 'E'))
   {
-    // Allow a negative exponent.
+    // Allow a single positive/negative exponent symbol.
+    if(!matchChar(parser, '+'))
+    {
       matchChar(parser, '-');
+    }
 
     if (!isDigit(peekChar(parser)))
     {

test/language/number/scientific_literals.wren

@@ -7,3 +7,4 @@ System.print(2.55e-2 is Num) // expect: true
 System.print(x is Num) // expect: true
 System.print(-2.55e2) // expect: -255
 System.print(-25500e-2) // expect: -255
+System.print(2.55e+2) // expect: 255

test/language/number/scientific_multiple_exponent_signs.wren

@@ -0,0 +1 @@
+var x = 1e+-01 // expect error