Bug Fix: win32 readFileContents() cannot read file names with unicode characters. (Fix utf8 to utf16)

[MVV90]

On Windows, running sqlite with sqlean extension: select fileio_read('hello世界.txt'); will fail silently, returning an empty string, regardless if hello世界.txt has contents.

Windows uses UTF16 for it's native file system encoding for all file names (versions NT, 2000, XP, 7, 10, 11). Files with unicode characters (eg: 世界 in hello世界.txt) would be converted to UTF8 internally to sqlean, when Windows is expecting a UTF16 encoding. This prevents sqlean from reading files on windows (that have unicode characters in the file name). Windows function _wfopen works with file names with or without unicode characters.

• Adds fix for win32 systems
• Add unit test coverage for reading file names with unicode characters in them.

[nalgeon]

Thank you for the PR!

Compilation on Windows is failing:

gcc -O3  -shared -Isrc -DSQLEAN_VERSION='"main"' src/sqlite3-fileio.c src/fileio/*.c -o dist/fileio.dll
src/fileio/legacy.c: In function 'readFileContents':
src/fileio/legacy.c:167:50: error: 'zPath' undeclared (first use in this function)
  167 |     zUnicodeName = sqlite3_win32_utf8_to_unicode(zPath);
      |                                                  ^~~~~
src/fileio/legacy.c:167:50: note: each undeclared identifier is reported only once for each function it appears in

[nalgeon]

Thanks!