SCI_SETDRAGDROPENABLED. Fully implemented on Win32 but may only prevent dragging
on other platforms.

Set optional SCINTILLA_BIN environment variable to point to build directory.
It's unclear just what the problem was but Python 3.13 would crash out in window
handling code when run inside the debugger and using its HINSTANCE.
Changing to a global class and using None for the HINSTANCE made it work.
There are various other minor problems here like using c_int (32-bit) for the
window procedure return instead of c_ssize_t (64-bit) but they are not worth
destabilizing the code further.

Example Scintilla.vcxproj.user for debugging a test script with a particular
Python interpreter.

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="Current" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|x64'">
    <LocalDebuggerCommand>C:\Users\Neil\AppData\Local\Programs\Python\Python313\python.exe</LocalDebuggerCommand>
    <DebuggerFlavor>WindowsLocalDebugger</DebuggerFlavor>
    <LocalDebuggerCommandArguments>G:\u\hg\scintilla\test\simpleTests.py</LocalDebuggerCommandArguments>
    <LocalDebuggerWorkingDirectory>G:\u\hg\scintilla\test\</LocalDebuggerWorkingDirectory>
    <LocalDebuggerEnvironment>SCINTILLA_BIN=G:\\u\\hg\\scintilla\\win32\\x64\\Debug</LocalDebuggerEnvironment>
  </PropertyGroup>
</Project>

Avoid processing characters multiple times by relying on from_chars munching
digit characters instead of searching for delimiters.

Add tests to check that inverted conversions yield the original value.

characters and invalid bytes.
Add more test cases.