Global str function only supports float arguments

[alimnios72]

Even when the Roku docs say that str takes a float argument, in reality it can take double values as well:

Brightscript Debugger> myvar = 1605743966.521#
Brightscript Debugger> print type(myvar)
Double
Brightscript Debugger> print str(myvar)
 1.605744e+09

The current implementation tries to enforce that float numbers are taken as arguments and it fails if a double is provided

 function Str(value as Float) as String:
        * Argument 'value' must be of type Float, but received Double.

We should match Roku implementation