change AsSpan to use a range from "from" to "end"

the current code assumes that float.Parse behaves the same as the
internal C++ code, however without using "end" as part of the span, it
will parse from index 0 to the end of the string, ignoring commas. for
example, this causes it to parse "0,5,0" with divisor "," as [50, 50,
0], as the float.Parse method ignores commas in floats.

if another divisor is used, it throws a System.FormatException due to
containing invalid characters, as it fails to account for the position
of the divisor for the span.

Author: PixelDough

modules/mono/glue/GodotSharp/GodotSharp/Core/StringExtensions.cs

@@ -1539,7 +1539,7 @@ public static float[] SplitFloats(this string instance, string divisor, bool all
                 if (end < 0)
                     end = len;
                 if (allowEmpty || end > from)
-                    ret.Add(float.Parse(instance.AsSpan(from), CultureInfo.InvariantCulture));
+                    ret.Add(float.Parse(instance.AsSpan(from, end - from), CultureInfo.InvariantCulture));
                 if (end == len)
                     break;