Issue 12898 - `std.process.browse` expects URL to be encoded in CP_ACP on Windows instead of UTF-8

Use Unicode version of `ShellExecute` in `std.process.browse`.
Also remove needless `ShellExecuteA` declaration.
Use Unicode version of `CreateFileMapping` in `std.mmfile.MmFile` constructor.

It's a bad style to use ANSI WinAPI function and may cause problems in future.