## Summary
Adds a unified mechanism for locating third-party IO DLLs through environment
variables, on top of the existing per-game `[xxxio] path=` INI setting.
## Changes
For each IO DLL, the path is resolved in this order:
1. The section-specific INI value, e.g. `[mai2io] path=`.
2. The section-specific environment variable, e.g. `SEGATOOLS_MAI2IO_PATH`.
3. `SEGATOOLS_IO_ROOT` joined with the default IO DLL filename, e.g.
`%SEGATOOLS_IO_ROOT%\mai2io.dll`.
4. Empty path, which keeps the built-in IO implementation (unchanged default).
INI and environment-variable paths are run through Windows environment-variable
expansion, so values such as `%SEGATOOLS_IO_ROOT%\mai2io.dll` are valid in the
INI directly.
Reviewed-on: https://gitea.tendokyu.moe/TeamTofuShop/segatools/pulls/119
Co-authored-by: グローランプ <130208311+Gl0w1amp@users.noreply.github.com>
Co-committed-by: グローランプ <130208311+Gl0w1amp@users.noreply.github.com>
